From 86a8979425a767ace21345fe63edf838eb5c381f Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Thu, 16 Apr 2026 20:36:56 -0700
Subject: [PATCH 01/20] Start work on an io_sender<Return(Args...)>

This diff starts the work to add a type-erased sender named
`io_sender<Return(Args...)>`. The intent is for such a sender to
represent "an async function from `Args...` to `Return`", a bit like a
task coroutine, but with different trade offs. The sender itself stores
a `std::tuple<Args...>` and a `sender auto(Args&&...)` factory that can
construct the intended erased sender from the stored arguments on
demand. This representation allows us to defer allocation of the
type-erased operation state until `connect` time, giving us
coroutine-like behaviour but allowing us to choose the frame allocator
by querying the eventual receiver's environment.

The completion signatures for an `io_sender<Return(Args...)>` are:
 - `set_value_t(R&&)`
 - `set_error_t(std::exception_ptr)`
 - `set_stopped_t()`

We may be able to eliminate the error channel for
`io_sender<R(A...) noexcept>` but that direction requires more thought.

This first diff proves that we can store a tuple of arguments and a
factory and, at `connect` time, use those values to allocate a
type-erased operation state. The test cases cover only basic cases, and
all allocations happen through `::operator new`. Future changes will
expand the test cases and invent a `get_frame_allocator` environment
query that can be used to control frame allocations. The expectation is
that we can meet Capy's performance characteristics with a slightly
different API in a sender-first way.
---
 include/exec/io/io_sender.hpp   | 301 ++++++++++++++++++++++++++++++++
 test/exec/CMakeLists.txt        |   1 +
 test/exec/io/test_io_sender.cpp |  54 ++++++
 3 files changed, 356 insertions(+)
 create mode 100644 include/exec/io/io_sender.hpp
 create mode 100644 test/exec/io/test_io_sender.cpp

diff --git a/include/exec/io/io_sender.hpp b/include/exec/io/io_sender.hpp
new file mode 100644
index 000000000..c77b3ef68
--- /dev/null
+++ b/include/exec/io/io_sender.hpp
@@ -0,0 +1,301 @@
+/* Copyright (c) 2026 Ian Petersen
+ * Copyright (c) 2026 NVIDIA Corporation
+ *
+ * Licensed under the Apache License Version 2.0 with LLVM Exceptions
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *   https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include "../../stdexec/__detail/__completion_signatures.hpp"
+#include "../../stdexec/__detail/__concepts.hpp"
+#include "../../stdexec/__detail/__env.hpp"
+#include "../../stdexec/__detail/__receivers.hpp"
+#include "../../stdexec/__detail/__sender_concepts.hpp"
+
+#include <exception>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+// This file defines io_sender<ReturnType(Arguments...)>, which is a
+// type-erased sender that can complete with
+//  - set_value(ReturnType&&)
+//  - set_error(std::exception_ptr)
+//  - set_stopped()
+//
+// The type-erased operation state is allocated in connect; to accomplish
+// this deferred allocation, the sender holds a tuple of arguments that
+// are passed into a sender-factory in connect, which is why the template
+// type parameter is a function type rather than just a return type.
+//
+// The intended use case is an ABI-stable API boundary, assuming that a
+// std::tuple<Arguments...> qualifies as "ABI-stable". The hope is that
+// this is a "better task" in that it represents an async function from
+// arguments to value, just like a task coroutine, but, by deferring the
+// allocation to connect, we can use receiver environment queries to pick
+// the frame allocator from the environment without relying on TLS.
+namespace experimental::execution
+{
+
+  // TODO: think about environment forwarding
+  template <class T, class Env = STDEXEC::env<>>
+  struct io_sender;
+
+  template <class R>
+  struct completer
+  {
+    completer() = default;
+
+    virtual void set_value(R&& value) noexcept = 0;
+
+    virtual void set_error(std::exception_ptr err) noexcept = 0;
+
+    virtual void set_stopped() noexcept = 0;
+
+   protected:
+    ~completer() = default;
+  };
+
+  template <>
+  struct completer<void>
+  {
+    completer() = default;
+
+    virtual void set_value() noexcept = 0;
+
+    virtual void set_error(std::exception_ptr err) noexcept = 0;
+
+    virtual void set_stopped() noexcept = 0;
+
+   protected:
+    ~completer() = default;
+  };
+
+  template <class R>
+  struct io_receiver
+  {
+    using receiver_concept = STDEXEC::receiver_tag;
+
+    void set_value(R&& value) noexcept
+    {
+      completer_->set_value(std::forward<R>(value));
+    }
+
+    void set_error(std::exception_ptr err) noexcept
+    {
+      completer_->set_error(std::move(err));
+    }
+
+    void set_stopped() noexcept
+    {
+      completer_->set_stopped();
+    }
+
+    completer<R>* completer_;
+  };
+
+  template <>
+  struct io_receiver<void>
+  {
+    using receiver_concept = STDEXEC::receiver_tag;
+
+    void set_value() noexcept
+    {
+      completer_->set_value();
+    }
+
+    void set_error(std::exception_ptr err) noexcept
+    {
+      completer_->set_error(std::move(err));
+    }
+
+    void set_stopped() noexcept
+    {
+      completer_->set_stopped();
+    }
+
+    completer<void>* completer_;
+  };
+
+  template <class Return>
+  struct io_sender_completions
+  {
+    template <class Sender, class /*Env*/>
+    static consteval STDEXEC::completion_signatures<STDEXEC::set_value_t(Return),
+                                                    STDEXEC::set_error_t(std::exception_ptr),
+                                                    STDEXEC::set_stopped_t()>
+    get_completion_signatures()
+    {
+      return {};
+    }
+  };
+
+  template <>
+  struct io_sender_completions<void>
+  {
+    template <class Sender, class /*Env*/>
+    static consteval STDEXEC::completion_signatures<STDEXEC::set_value_t(),
+                                                    STDEXEC::set_error_t(std::exception_ptr),
+                                                    STDEXEC::set_stopped_t()>
+    get_completion_signatures()
+    {
+      return {};
+    }
+  };
+
+  struct base_operation
+  {
+    base_operation()                 = default;
+    base_operation(base_operation&&) = delete;
+    virtual ~base_operation()        = default;
+
+    virtual void start() & noexcept = 0;
+  };
+
+  template <class R, class Receiver>
+  struct operation_storage : completer<R>
+  {
+    explicit operation_storage(Receiver rcvr) noexcept
+      : receiver_(std::move(rcvr))
+    {}
+
+    void set_value(R&& value) noexcept final
+    {
+      STDEXEC::set_value(std::move(receiver_), std::forward<R>(value));
+    }
+
+    Receiver receiver_;
+  };
+
+  template <class Receiver>
+  struct operation_storage<void, Receiver> : completer<void>
+  {
+    explicit operation_storage(Receiver rcvr) noexcept
+      : receiver_(std::move(rcvr))
+    {}
+
+    void set_value() noexcept final
+    {
+      STDEXEC::set_value(std::move(receiver_));
+    }
+
+    Receiver receiver_;
+  };
+
+  template <class R, class Receiver>
+  struct operation : operation_storage<R, Receiver>
+  {
+    using operation_state_concept = STDEXEC::operation_state_tag;
+
+    template <class Factory>
+    operation(Receiver rcvr, Factory factory)
+      : operation_storage<R, Receiver>{std::move(rcvr)}
+      , op_(factory(io_receiver<R>(this)))
+    {}
+
+    void start() & noexcept
+    {
+      op_->start();
+    }
+
+   private:
+    std::unique_ptr<base_operation> op_;
+
+    void set_error(std::exception_ptr err) noexcept final
+    {
+      STDEXEC::set_error(std::move(this->receiver_), std::move(err));
+    }
+
+    void set_stopped() noexcept final
+    {
+      STDEXEC::set_stopped(std::move(this->receiver_));
+    }
+  };
+
+  // consider:
+  //
+  //   template <class R, class... Args>
+  //   struct io_sender<R(Args...) noexcept> {};
+  //
+  // to declare no error channel
+  //
+  // we allocate in connect, which could throw, but that just means connect
+  // can't be noexcept; it doesn't mean we have to have an error channel after
+  // we successfully connect...
+  template <class R, class... Args, class Env>
+    requires((std::movable<Args> || std::is_reference_v<Args>) && ...)
+  struct io_sender<R(Args...), Env> : io_sender_completions<R>
+  {
+    using sender_concept = STDEXEC::sender_tag;
+
+    template <STDEXEC::__callable<Args...> Factory>
+      requires STDEXEC::__not_decays_to<Factory, io_sender>  //
+            && std::constructible_from<Factory>              //
+            && STDEXEC::__callable<Factory, Args...>
+            && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>, io_receiver<R>>
+    explicit(sizeof...(Args) == 0) io_sender(Args&&... args, Factory&& factory)
+      noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
+      : args_(std::forward<Args>(args)...)
+    {
+      using sender_t = std::invoke_result_t<Factory, Args...>;
+
+      struct derived_operation : base_operation
+      {
+        explicit derived_operation(sender_t&& sndr, io_receiver<R> rcvr)  // TODO noexcept
+          : op_(STDEXEC::connect(std::forward<sender_t>(sndr), std::move(rcvr)))
+        {}
+
+        ~derived_operation() override = default;
+
+        void start() & noexcept override
+        {
+          STDEXEC::start(op_);
+        }
+
+       private:
+        STDEXEC::connect_result_t<sender_t, io_receiver<R>> op_;
+      };
+
+      factory_ = [](io_receiver<R> rcvr, Args&&... args) -> base_operation*
+      {
+        Factory factory;
+        // TODO: query rcvr for a frame allocator and use it
+        return new derived_operation(factory(std::forward<Args>(args)...), std::move(rcvr));
+      };
+    }
+
+    template <class Self, STDEXEC::receiver Receiver>
+    auto connect(this Self&& sender, Receiver receiver) -> operation<R, Receiver>
+    {
+      return operation<R, Receiver>(std::move(receiver),
+                                    [&](io_receiver<R> rcvr)
+                                    {
+                                      return std::apply(
+                                        [&](Args&&... args)
+                                        {
+                                          return sender.factory_(std::move(rcvr),
+                                                                 std::forward<Args>(args)...);
+                                        },
+                                        std::forward<Self>(sender).args_);
+                                    });
+    }
+
+   private:
+    base_operation* (*factory_)(io_receiver<R>, Args&&...);
+    [[no_unique_address]]
+    std::tuple<Args...> args_;
+  };
+
+}  // namespace experimental::execution
+
+namespace exec = experimental::execution;
diff --git a/test/exec/CMakeLists.txt b/test/exec/CMakeLists.txt
index 93a30070a..4cdbdfa25 100644
--- a/test/exec/CMakeLists.txt
+++ b/test/exec/CMakeLists.txt
@@ -61,6 +61,7 @@ set(exec_test_sources
     $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:sequence/test_merge_each_threaded.cpp>
     $<$<BOOL:${STDEXEC_ENABLE_LIBDISPATCH}>:test_libdispatch.cpp>
     test_unless_stop_requested.cpp
+    io/test_io_sender.cpp
     )
 
 set_source_files_properties(test_any_sender.cpp
diff --git a/test/exec/io/test_io_sender.cpp b/test/exec/io/test_io_sender.cpp
new file mode 100644
index 000000000..6aadc6340
--- /dev/null
+++ b/test/exec/io/test_io_sender.cpp
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2026 Ian Petersen
+ * Copyright (c) 2026 NVIDIA Corporation
+ *
+ * Licensed under the Apache License Version 2.0 with LLVM Exceptions
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *   https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <exec/io/io_sender.hpp>
+
+#include <catch2/catch_all.hpp>
+
+#include <stdexec/execution.hpp>
+
+namespace ex = STDEXEC;
+
+namespace
+{
+
+  TEST_CASE("exec::io_sender is constructible", "[types][io_sender]")
+  {
+    exec::io_sender<void()> voidSndr([]() noexcept { return ex::just(); });
+
+    exec::io_sender<int()> intSndr([]() noexcept { return ex::just(42); });
+
+    double                              d = 4.;
+    exec::io_sender<void(int, double&)> binarySndr(5,
+                                                   d,
+                                                   [](int, double&) noexcept
+                                                   { return ex::just(); });
+
+    STATIC_REQUIRE(STDEXEC::sender<decltype(voidSndr)>);
+    STATIC_REQUIRE(STDEXEC::sender<decltype(intSndr)>);
+    STATIC_REQUIRE(STDEXEC::sender<decltype(binarySndr)>);
+  }
+
+  TEST_CASE("exec::io_sender is connectable", "[types][io_sender]")
+  {
+    exec::io_sender<int()> sndr([]() noexcept { return ex::just(42); });
+
+    auto [fortytwo] = ex::sync_wait(std::move(sndr)).value();
+
+    REQUIRE(fortytwo == 42);
+  }
+}  // namespace

From 3b5911bcd0dfca410a11fc5374a605a59cacce48 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Fri, 17 Apr 2026 16:46:52 -0700
Subject: [PATCH 02/20] Rename io_sender to function

This diff changes the name of `io_sender<R(A...)>` to
`function<R(A...)>` after some discussion with other folks working on
`std::execution`. `exec::function<...>` is a type-erased wrapper around
an async function with the given signature (elided here as `...`). More
features are coming in future diffs.
---
 .../exec/{io/io_sender.hpp => function.hpp}   | 47 ++++++++++---------
 test/exec/CMakeLists.txt                      |  2 +-
 .../test_io_sender.cpp => test_function.cpp}  | 21 ++++-----
 3 files changed, 35 insertions(+), 35 deletions(-)
 rename include/exec/{io/io_sender.hpp => function.hpp} (85%)
 rename test/exec/{io/test_io_sender.cpp => test_function.cpp} (60%)

diff --git a/include/exec/io/io_sender.hpp b/include/exec/function.hpp
similarity index 85%
rename from include/exec/io/io_sender.hpp
rename to include/exec/function.hpp
index c77b3ef68..a75314366 100644
--- a/include/exec/io/io_sender.hpp
+++ b/include/exec/function.hpp
@@ -15,11 +15,11 @@
  */
 #pragma once
 
-#include "../../stdexec/__detail/__completion_signatures.hpp"
-#include "../../stdexec/__detail/__concepts.hpp"
-#include "../../stdexec/__detail/__env.hpp"
-#include "../../stdexec/__detail/__receivers.hpp"
-#include "../../stdexec/__detail/__sender_concepts.hpp"
+#include "../stdexec/__detail/__completion_signatures.hpp"
+#include "../stdexec/__detail/__concepts.hpp"
+#include "../stdexec/__detail/__env.hpp"
+#include "../stdexec/__detail/__receivers.hpp"
+#include "../stdexec/__detail/__sender_concepts.hpp"
 
 #include <exception>
 #include <memory>
@@ -27,7 +27,7 @@
 #include <type_traits>
 #include <utility>
 
-// This file defines io_sender<ReturnType(Arguments...)>, which is a
+// This file defines function<ReturnType(Arguments...)>, which is a
 // type-erased sender that can complete with
 //  - set_value(ReturnType&&)
 //  - set_error(std::exception_ptr)
@@ -49,7 +49,7 @@ namespace experimental::execution
 
   // TODO: think about environment forwarding
   template <class T, class Env = STDEXEC::env<>>
-  struct io_sender;
+  struct function;
 
   template <class R>
   struct completer
@@ -82,7 +82,7 @@ namespace experimental::execution
   };
 
   template <class R>
-  struct io_receiver
+  struct function_receiver
   {
     using receiver_concept = STDEXEC::receiver_tag;
 
@@ -105,7 +105,7 @@ namespace experimental::execution
   };
 
   template <>
-  struct io_receiver<void>
+  struct function_receiver<void>
   {
     using receiver_concept = STDEXEC::receiver_tag;
 
@@ -128,7 +128,7 @@ namespace experimental::execution
   };
 
   template <class Return>
-  struct io_sender_completions
+  struct function_completions
   {
     template <class Sender, class /*Env*/>
     static consteval STDEXEC::completion_signatures<STDEXEC::set_value_t(Return),
@@ -141,7 +141,7 @@ namespace experimental::execution
   };
 
   template <>
-  struct io_sender_completions<void>
+  struct function_completions<void>
   {
     template <class Sender, class /*Env*/>
     static consteval STDEXEC::completion_signatures<STDEXEC::set_value_t(),
@@ -200,7 +200,7 @@ namespace experimental::execution
     template <class Factory>
     operation(Receiver rcvr, Factory factory)
       : operation_storage<R, Receiver>{std::move(rcvr)}
-      , op_(factory(io_receiver<R>(this)))
+      , op_(factory(function_receiver<R>(this)))
     {}
 
     void start() & noexcept
@@ -225,7 +225,7 @@ namespace experimental::execution
   // consider:
   //
   //   template <class R, class... Args>
-  //   struct io_sender<R(Args...) noexcept> {};
+  //   struct function<R(Args...) noexcept> {};
   //
   // to declare no error channel
   //
@@ -234,16 +234,17 @@ namespace experimental::execution
   // we successfully connect...
   template <class R, class... Args, class Env>
     requires((std::movable<Args> || std::is_reference_v<Args>) && ...)
-  struct io_sender<R(Args...), Env> : io_sender_completions<R>
+  struct function<R(Args...), Env> : function_completions<R>
   {
     using sender_concept = STDEXEC::sender_tag;
 
     template <STDEXEC::__callable<Args...> Factory>
-      requires STDEXEC::__not_decays_to<Factory, io_sender>  //
-            && std::constructible_from<Factory>              //
+      requires STDEXEC::__not_decays_to<Factory, function>  //
+            && std::constructible_from<Factory>             //
             && STDEXEC::__callable<Factory, Args...>
-            && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>, io_receiver<R>>
-    explicit(sizeof...(Args) == 0) io_sender(Args&&... args, Factory&& factory)
+            && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
+                                  function_receiver<R>>
+    explicit(sizeof...(Args) == 0) function(Args&&... args, Factory&& factory)
       noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
       : args_(std::forward<Args>(args)...)
     {
@@ -251,7 +252,7 @@ namespace experimental::execution
 
       struct derived_operation : base_operation
       {
-        explicit derived_operation(sender_t&& sndr, io_receiver<R> rcvr)  // TODO noexcept
+        explicit derived_operation(sender_t&& sndr, function_receiver<R> rcvr)  // TODO noexcept
           : op_(STDEXEC::connect(std::forward<sender_t>(sndr), std::move(rcvr)))
         {}
 
@@ -263,10 +264,10 @@ namespace experimental::execution
         }
 
        private:
-        STDEXEC::connect_result_t<sender_t, io_receiver<R>> op_;
+        STDEXEC::connect_result_t<sender_t, function_receiver<R>> op_;
       };
 
-      factory_ = [](io_receiver<R> rcvr, Args&&... args) -> base_operation*
+      factory_ = [](function_receiver<R> rcvr, Args&&... args) -> base_operation*
       {
         Factory factory;
         // TODO: query rcvr for a frame allocator and use it
@@ -278,7 +279,7 @@ namespace experimental::execution
     auto connect(this Self&& sender, Receiver receiver) -> operation<R, Receiver>
     {
       return operation<R, Receiver>(std::move(receiver),
-                                    [&](io_receiver<R> rcvr)
+                                    [&](function_receiver<R> rcvr)
                                     {
                                       return std::apply(
                                         [&](Args&&... args)
@@ -291,7 +292,7 @@ namespace experimental::execution
     }
 
    private:
-    base_operation* (*factory_)(io_receiver<R>, Args&&...);
+    base_operation* (*factory_)(function_receiver<R>, Args&&...);
     [[no_unique_address]]
     std::tuple<Args...> args_;
   };
diff --git a/test/exec/CMakeLists.txt b/test/exec/CMakeLists.txt
index 4cdbdfa25..388143fad 100644
--- a/test/exec/CMakeLists.txt
+++ b/test/exec/CMakeLists.txt
@@ -61,7 +61,7 @@ set(exec_test_sources
     $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:sequence/test_merge_each_threaded.cpp>
     $<$<BOOL:${STDEXEC_ENABLE_LIBDISPATCH}>:test_libdispatch.cpp>
     test_unless_stop_requested.cpp
-    io/test_io_sender.cpp
+    test_function.cpp
     )
 
 set_source_files_properties(test_any_sender.cpp
diff --git a/test/exec/io/test_io_sender.cpp b/test/exec/test_function.cpp
similarity index 60%
rename from test/exec/io/test_io_sender.cpp
rename to test/exec/test_function.cpp
index 6aadc6340..dff7c5d64 100644
--- a/test/exec/io/test_io_sender.cpp
+++ b/test/exec/test_function.cpp
@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-#include <exec/io/io_sender.hpp>
+#include <exec/function.hpp>
 
 #include <catch2/catch_all.hpp>
 
@@ -26,26 +26,25 @@ namespace ex = STDEXEC;
 namespace
 {
 
-  TEST_CASE("exec::io_sender is constructible", "[types][io_sender]")
+  TEST_CASE("exec::function is constructible", "[types][function]")
   {
-    exec::io_sender<void()> voidSndr([]() noexcept { return ex::just(); });
+    exec::function<void()> voidSndr([]() noexcept { return ex::just(); });
 
-    exec::io_sender<int()> intSndr([]() noexcept { return ex::just(42); });
+    exec::function<int()> intSndr([]() noexcept { return ex::just(42); });
 
-    double                              d = 4.;
-    exec::io_sender<void(int, double&)> binarySndr(5,
-                                                   d,
-                                                   [](int, double&) noexcept
-                                                   { return ex::just(); });
+    double                             d = 4.;
+    exec::function<void(int, double&)> binarySndr(5,
+                                                  d,
+                                                  [](int, double&) noexcept { return ex::just(); });
 
     STATIC_REQUIRE(STDEXEC::sender<decltype(voidSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(intSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(binarySndr)>);
   }
 
-  TEST_CASE("exec::io_sender is connectable", "[types][io_sender]")
+  TEST_CASE("exec::function is connectable", "[types][function]")
   {
-    exec::io_sender<int()> sndr([]() noexcept { return ex::just(42); });
+    exec::function<int()> sndr([]() noexcept { return ex::just(42); });
 
     auto [fortytwo] = ex::sync_wait(std::move(sndr)).value();
 

From 1ea9036b759e5d716305a261f28d0ad931de8f59 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Fri, 17 Apr 2026 20:06:48 -0700
Subject: [PATCH 03/20] Generalize implementation

Move to an implementation that spreads `completion_signatures`
throughout the internals so that we're not restricted to `R(A...)`-style
constraints.

The tests still only validate `R(A...)`-style constraints, with no
validation of no-throw functions, or controlling the completion
signature and environment; that'll come next.

This implementation also relies on virtual inheritance of a pack of
abstract base classes, which feels like a kludge. I should figure out
how to reimplement the virtual dispatch in terms of a hand-rolled
vtable.
---
 include/exec/function.hpp | 449 ++++++++++++++++++++++----------------
 1 file changed, 255 insertions(+), 194 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index a75314366..ffa0a6770 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -46,257 +46,318 @@
 // the frame allocator from the environment without relying on TLS.
 namespace experimental::execution
 {
+  namespace _func
+  {
+    using namespace STDEXEC;
 
-  // TODO: think about environment forwarding
-  template <class T, class Env = STDEXEC::env<>>
-  struct function;
+    template <class Sig>
+    struct _virt_completion;
 
-  template <class R>
-  struct completer
-  {
-    completer() = default;
+    template <class Error>
+    struct _virt_completion<set_error_t(Error)>
+    {
+      _virt_completion() = default;
 
-    virtual void set_value(R&& value) noexcept = 0;
+      _virt_completion(_virt_completion&&) = delete;
 
-    virtual void set_error(std::exception_ptr err) noexcept = 0;
+      virtual void set_error(Error&& err) noexcept = 0;
 
-    virtual void set_stopped() noexcept = 0;
+     protected:
+      ~_virt_completion() = default;
+    };
 
-   protected:
-    ~completer() = default;
-  };
+    template <>
+    struct _virt_completion<set_stopped_t()>
+    {
+      _virt_completion() = default;
 
-  template <>
-  struct completer<void>
-  {
-    completer() = default;
+      _virt_completion(_virt_completion&&) = delete;
+
+      virtual void set_stopped() noexcept = 0;
 
-    virtual void set_value() noexcept = 0;
+     protected:
+      ~_virt_completion() = default;
+    };
 
-    virtual void set_error(std::exception_ptr err) noexcept = 0;
+    template <class... Values>
+    struct _virt_completion<set_value_t(Values...)>
+    {
+      _virt_completion() = default;
 
-    virtual void set_stopped() noexcept = 0;
+      _virt_completion(_virt_completion&&) = delete;
 
-   protected:
-    ~completer() = default;
-  };
+      virtual void set_value(Values&&... values) noexcept = 0;
 
-  template <class R>
-  struct function_receiver
-  {
-    using receiver_concept = STDEXEC::receiver_tag;
+     protected:
+      ~_virt_completion() = default;
+    };
 
-    void set_value(R&& value) noexcept
-    {
-      completer_->set_value(std::forward<R>(value));
-    }
+    template <class Sigs>
+    struct _virt_completions;
 
-    void set_error(std::exception_ptr err) noexcept
+    template <class... Sigs>
+    struct _virt_completions<completion_signatures<Sigs...>> : virtual _virt_completion<Sigs>...
     {
-      completer_->set_error(std::move(err));
-    }
+      _virt_completions() = default;
 
-    void set_stopped() noexcept
-    {
-      completer_->set_stopped();
-    }
+      _virt_completions(_virt_completions&&) = delete;
 
-    completer<R>* completer_;
-  };
+     protected:
+      ~_virt_completions() = default;
+    };
 
-  template <>
-  struct function_receiver<void>
-  {
-    using receiver_concept = STDEXEC::receiver_tag;
+    template <class Sig, class Derived>
+    struct _func_rcvr_base;
 
-    void set_value() noexcept
+    template <class Error, class Derived>
+    struct _func_rcvr_base<set_error_t(Error), Derived>
     {
-      completer_->set_value();
-    }
+      void set_error(Error&& err) && noexcept
+      {
+        static_cast<Derived*>(this)->completer_->set_error(std::forward<Error>(err));
+      }
+    };
 
-    void set_error(std::exception_ptr err) noexcept
+    template <class Derived>
+    struct _func_rcvr_base<set_stopped_t(), Derived>
     {
-      completer_->set_error(std::move(err));
-    }
+      void set_stopped() && noexcept
+      {
+        static_cast<Derived*>(this)->completer_->set_stopped();
+      }
+    };
 
-    void set_stopped() noexcept
+    template <class... Value, class Derived>
+    struct _func_rcvr_base<set_value_t(Value...), Derived>
     {
-      completer_->set_stopped();
-    }
+      void set_value(Value&&... value) && noexcept
+      {
+        static_cast<Derived*>(this)->completer_->set_value(std::forward<Value>(value)...);
+      }
+    };
 
-    completer<void>* completer_;
-  };
+    template <class Sigs>
+    class _func_rcvr;
 
-  template <class Return>
-  struct function_completions
-  {
-    template <class Sender, class /*Env*/>
-    static consteval STDEXEC::completion_signatures<STDEXEC::set_value_t(Return),
-                                                    STDEXEC::set_error_t(std::exception_ptr),
-                                                    STDEXEC::set_stopped_t()>
-    get_completion_signatures()
+    template <class... Sigs>
+    class _func_rcvr<completion_signatures<Sigs...>>
+      : public _func_rcvr_base<Sigs, _func_rcvr<completion_signatures<Sigs...>>>...
     {
-      return {};
-    }
-  };
+      friend _func_rcvr_base<Sigs, _func_rcvr>...;
 
-  template <>
-  struct function_completions<void>
-  {
-    template <class Sender, class /*Env*/>
-    static consteval STDEXEC::completion_signatures<STDEXEC::set_value_t(),
-                                                    STDEXEC::set_error_t(std::exception_ptr),
-                                                    STDEXEC::set_stopped_t()>
-    get_completion_signatures()
-    {
-      return {};
-    }
-  };
+      using completer_t = _virt_completions<completion_signatures<Sigs...>>;
 
-  struct base_operation
-  {
-    base_operation()                 = default;
-    base_operation(base_operation&&) = delete;
-    virtual ~base_operation()        = default;
+      completer_t* completer_;
 
-    virtual void start() & noexcept = 0;
-  };
+     public:
+      using receiver_concept = receiver_tag;
 
-  template <class R, class Receiver>
-  struct operation_storage : completer<R>
-  {
-    explicit operation_storage(Receiver rcvr) noexcept
-      : receiver_(std::move(rcvr))
-    {}
+      explicit _func_rcvr(completer_t& completer) noexcept
+        : completer_(std::addressof(completer))
+      {}
+
+      // TODO: get_env
+    };
 
-    void set_value(R&& value) noexcept final
+    struct _base_op
     {
-      STDEXEC::set_value(std::move(receiver_), std::forward<R>(value));
-    }
+      _base_op() = default;
 
-    Receiver receiver_;
-  };
+      _base_op(_base_op&&) = delete;
 
-  template <class Receiver>
-  struct operation_storage<void, Receiver> : completer<void>
-  {
-    explicit operation_storage(Receiver rcvr) noexcept
-      : receiver_(std::move(rcvr))
-    {}
+      virtual ~_base_op() = default;
 
-    void set_value() noexcept final
+      virtual void start() & noexcept = 0;
+    };
+
+    template <class Sender, class Receiver>
+    struct _derived_op : _base_op
     {
-      STDEXEC::set_value(std::move(receiver_));
-    }
+      explicit _derived_op(Sender&& sndr, Receiver rcvr)
+        noexcept(std::is_nothrow_invocable_v<connect_t, Sender, Receiver>)
+        : op_(connect(std::forward<Sender>(sndr), std::move(rcvr)))
+      {}
 
-    Receiver receiver_;
-  };
+      _derived_op(_derived_op&&) = delete;
 
-  template <class R, class Receiver>
-  struct operation : operation_storage<R, Receiver>
-  {
-    using operation_state_concept = STDEXEC::operation_state_tag;
+      ~_derived_op() override = default;
 
-    template <class Factory>
-    operation(Receiver rcvr, Factory factory)
-      : operation_storage<R, Receiver>{std::move(rcvr)}
-      , op_(factory(function_receiver<R>(this)))
-    {}
+      void start() & noexcept override
+      {
+        ::STDEXEC::start(op_);
+      }
 
-    void start() & noexcept
-    {
-      op_->start();
-    }
+     private:
+      connect_result_t<Sender, Receiver> op_;
+    };
 
-   private:
-    std::unique_ptr<base_operation> op_;
+    template <class Sig, class Derived>
+    struct _func_op_completion;
 
-    void set_error(std::exception_ptr err) noexcept final
+    template <class Error, class Derived>
+    struct _func_op_completion<set_error_t(Error), Derived>
+      : virtual _virt_completion<set_error_t(Error)>
     {
-      STDEXEC::set_error(std::move(this->receiver_), std::move(err));
-    }
+      void set_error(Error&& err) noexcept final
+      {
+        static_cast<Derived*>(this)->complete(set_error_t{}, std::forward<Error>(err));
+      }
+    };
 
-    void set_stopped() noexcept final
+    template <class Derived>
+    struct _func_op_completion<set_stopped_t(), Derived> : virtual _virt_completion<set_stopped_t()>
     {
-      STDEXEC::set_stopped(std::move(this->receiver_));
-    }
-  };
+      void set_stopped() noexcept final
+      {
+        static_cast<Derived*>(this)->complete(set_stopped_t{});
+      }
+    };
 
-  // consider:
-  //
-  //   template <class R, class... Args>
-  //   struct function<R(Args...) noexcept> {};
-  //
-  // to declare no error channel
-  //
-  // we allocate in connect, which could throw, but that just means connect
-  // can't be noexcept; it doesn't mean we have to have an error channel after
-  // we successfully connect...
-  template <class R, class... Args, class Env>
-    requires((std::movable<Args> || std::is_reference_v<Args>) && ...)
-  struct function<R(Args...), Env> : function_completions<R>
-  {
-    using sender_concept = STDEXEC::sender_tag;
-
-    template <STDEXEC::__callable<Args...> Factory>
-      requires STDEXEC::__not_decays_to<Factory, function>  //
-            && std::constructible_from<Factory>             //
-            && STDEXEC::__callable<Factory, Args...>
-            && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
-                                  function_receiver<R>>
-    explicit(sizeof...(Args) == 0) function(Args&&... args, Factory&& factory)
-      noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
-      : args_(std::forward<Args>(args)...)
+    template <class... Value, class Derived>
+    struct _func_op_completion<set_value_t(Value...), Derived>
+      : virtual _virt_completion<set_value_t(Value...)>
+    {
+      void set_value(Value&&... value) noexcept final
+      {
+        static_cast<Derived*>(this)->complete(set_value_t{}, std::forward<Value>(value)...);
+      }
+    };
+
+    template <class Receiver, class Sigs>
+    class _func_op;
+
+    template <class Receiver, class... Sigs>
+    class _func_op<Receiver, completion_signatures<Sigs...>>
+      : private _virt_completions<completion_signatures<Sigs...>>
+      , private _func_op_completion<Sigs, _func_op<Receiver, completion_signatures<Sigs...>>>...
     {
-      using sender_t = std::invoke_result_t<Factory, Args...>;
+      // TODO: use get_frame_allocator(get_env(rcvr_)) to allocate and destroy this
+      std::unique_ptr<_base_op> op_;
+      [[no_unique_address]]
+      Receiver rcvr_;
+
+      friend _func_op_completion<Sigs, _func_op>...;
 
-      struct derived_operation : base_operation
+      template <class CPO, class... Arg>
+      void complete(CPO cpo, Arg&&... arg) noexcept
       {
-        explicit derived_operation(sender_t&& sndr, function_receiver<R> rcvr)  // TODO noexcept
-          : op_(STDEXEC::connect(std::forward<sender_t>(sndr), std::move(rcvr)))
-        {}
+        std::move(cpo)(std::move(rcvr_), std::forward<Arg>(arg)...);
+      }
 
-        ~derived_operation() override = default;
+     public:
+      using operation_state_concept = operation_state_tag;
 
-        void start() & noexcept override
-        {
-          STDEXEC::start(op_);
-        }
+      template <class Factory>
+      _func_op(Receiver rcvr, Factory factory)
+        : rcvr_(std::move(rcvr))
+        , op_(factory(_func_rcvr<completion_signatures<Sigs...>>(*this))){};
+
+      _func_op(_func_op&&) = delete;
+
+      ~_func_op() = default;
+
+      void start() & noexcept
+      {
+        op_->start();
+      }
+    };
+
+    template <class Args, class Sigs, class Env>
+    class _func_impl;
+
+    template <class SndrCncpt, class... Args, class... Sigs, class Env>
+    class _func_impl<SndrCncpt(Args...), completion_signatures<Sigs...>, Env>
+    {
+      _base_op* (*factory_)(_func_rcvr<completion_signatures<Sigs...>>, Args&&...);
+      [[no_unique_address]]
+      std::tuple<Args...> args_;
+
+     public:
+      using sender_concept = SndrCncpt;
+
+      template <STDEXEC::__callable<Args...> Factory>
+        requires STDEXEC::__not_decays_to<Factory, _func_impl>  //
+              && std::constructible_from<Factory>               //
+              && STDEXEC::__callable<Factory, Args...>
+      //&& STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
+      //_func_rcvr<completion_signatures<Sigs...>>>
+      explicit(sizeof...(Args) == 0) _func_impl(Args&&... args, Factory&& factory)
+        noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
+        : args_(std::forward<Args>(args)...)
+      {
+        using sender_t   = std::invoke_result_t<Factory, Args...>;
+        using receiver_t = _func_rcvr<completion_signatures<Sigs...>>;
+
+        using op_t = _derived_op<sender_t, receiver_t>;
 
-       private:
-        STDEXEC::connect_result_t<sender_t, function_receiver<R>> op_;
-      };
+        factory_ = [](receiver_t rcvr, Args&&... args) -> _base_op*
+        {
+          Factory factory;
+          // TODO: query rcvr for a frame allocator and use it
+          return new op_t(factory(std::forward<Args>(args)...), std::move(rcvr));
+        };
+      }
+
+      template <class Sender, class /* Env */>
+      static consteval completion_signatures<Sigs...> get_completion_signatures() noexcept
+      {
+        // TODO: validate that the Env passed here is compatible with the class-level Env
+        return {};
+      }
 
-      factory_ = [](function_receiver<R> rcvr, Args&&... args) -> base_operation*
+      template <class Receiver>
+      constexpr _func_op<Receiver, completion_signatures<Sigs...>> connect(Receiver rcvr)
       {
-        Factory factory;
-        // TODO: query rcvr for a frame allocator and use it
-        return new derived_operation(factory(std::forward<Args>(args)...), std::move(rcvr));
-      };
-    }
-
-    template <class Self, STDEXEC::receiver Receiver>
-    auto connect(this Self&& sender, Receiver receiver) -> operation<R, Receiver>
+        return {std::move(rcvr),
+                [&, this](auto rcvr)
+                {
+                  return std::apply(
+                    [&](Args&&... args)
+                    { return factory_(std::move(rcvr), std::forward<Args>(args)...); },
+                    std::move(args_));
+                }};
+      }
+    };
+
+    template <class Sig>
+    struct _sigs_from;
+
+    template <class Return, class... Args>
+    struct _sigs_from<Return(Args...)>
     {
-      return operation<R, Receiver>(std::move(receiver),
-                                    [&](function_receiver<R> rcvr)
-                                    {
-                                      return std::apply(
-                                        [&](Args&&... args)
-                                        {
-                                          return sender.factory_(std::move(rcvr),
-                                                                 std::forward<Args>(args)...);
-                                        },
-                                        std::forward<Self>(sender).args_);
-                                    });
-    }
-
-   private:
-    base_operation* (*factory_)(function_receiver<R>, Args&&...);
-    [[no_unique_address]]
-    std::tuple<Args...> args_;
-  };
+      using type = STDEXEC::completion_signatures<STDEXEC::set_error_t(std::exception_ptr),
+                                                  STDEXEC::set_stopped_t(),
+                                                  STDEXEC::set_value_t(Return)>;
+    };
+
+    template <class... Args>
+    struct _sigs_from<void(Args...)>
+    {
+      using type = STDEXEC::completion_signatures<STDEXEC::set_error_t(std::exception_ptr),
+                                                  STDEXEC::set_stopped_t(),
+                                                  STDEXEC::set_value_t()>;
+    };
+
+    template <class Sig>
+    using _sigs_from_t = _sigs_from<Sig>::type;
+  }  // namespace _func
 
+  // TODO: think about environment forwarding
+  template <class...>
+  class function;
+
+  template <class Return, class... Args>
+  class function<Return(Args...)>
+    : public _func::_func_impl<STDEXEC::sender_tag(Args...),
+                               _func::_sigs_from_t<Return(Args...)>,
+                               STDEXEC::env<>>
+  {
+    using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
+                                   _func::_sigs_from_t<Return(Args...)>,
+                                   STDEXEC::env<>>;
+
+    using base::base;
+  };
 }  // namespace experimental::execution
 
 namespace exec = experimental::execution;

From fd531388c26767b21b9ff6363fbde4a643c88f1b Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sat, 18 Apr 2026 09:31:23 -0700
Subject: [PATCH 04/20] Support no-throw functions

---
 include/exec/function.hpp   | 21 +++++++++++++++++----
 test/exec/test_function.cpp |  5 +++++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index ffa0a6770..4c2a682e0 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -338,6 +338,19 @@ namespace experimental::execution
                                                   STDEXEC::set_value_t()>;
     };
 
+    template <class Return, class... Args>
+    struct _sigs_from<Return(Args...) noexcept>
+    {
+      using type =
+        STDEXEC::completion_signatures<STDEXEC::set_stopped_t(), STDEXEC::set_value_t(Return)>;
+    };
+
+    template <class... Args>
+    struct _sigs_from<void(Args...) noexcept>
+    {
+      using type = STDEXEC::completion_signatures<STDEXEC::set_stopped_t(), STDEXEC::set_value_t()>;
+    };
+
     template <class Sig>
     using _sigs_from_t = _sigs_from<Sig>::type;
   }  // namespace _func
@@ -346,14 +359,14 @@ namespace experimental::execution
   template <class...>
   class function;
 
-  template <class Return, class... Args>
-  class function<Return(Args...)>
+  template <class Return, class... Args, bool NoThrow>
+  class function<Return(Args...) noexcept(NoThrow)>
     : public _func::_func_impl<STDEXEC::sender_tag(Args...),
-                               _func::_sigs_from_t<Return(Args...)>,
+                               _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
                                STDEXEC::env<>>
   {
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
-                                   _func::_sigs_from_t<Return(Args...)>,
+                                   _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
                                    STDEXEC::env<>>;
 
     using base::base;
diff --git a/test/exec/test_function.cpp b/test/exec/test_function.cpp
index dff7c5d64..b855b38c0 100644
--- a/test/exec/test_function.cpp
+++ b/test/exec/test_function.cpp
@@ -37,9 +37,14 @@ namespace
                                                   d,
                                                   [](int, double&) noexcept { return ex::just(); });
 
+    exec::function<void() noexcept> nothrowSndr([]() noexcept { return ex::just(); });
+    exec::function<int() noexcept>  nothrowIntSndr([]() noexcept { return ex::just(42); });
+
     STATIC_REQUIRE(STDEXEC::sender<decltype(voidSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(intSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(binarySndr)>);
+    STATIC_REQUIRE(STDEXEC::sender<decltype(nothrowSndr)>);
+    STATIC_REQUIRE(STDEXEC::sender<decltype(nothrowIntSndr)>);
   }
 
   TEST_CASE("exec::function is connectable", "[types][function]")

From 665566a853a3a4e8efd122413b5aefc06c608e88 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sat, 18 Apr 2026 20:44:18 -0700
Subject: [PATCH 05/20] Get rid of virtual inheritance

Thanks to a suggestion from @RobertLeahy, I've been able to rework the
virtual function inheritance to not need virtual inheritance.
---
 include/exec/function.hpp   | 140 ++++++++++++------------------------
 test/exec/test_function.cpp |  17 ++++-
 2 files changed, 60 insertions(+), 97 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 4c2a682e0..9a62f4dc2 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -53,40 +53,14 @@ namespace experimental::execution
     template <class Sig>
     struct _virt_completion;
 
-    template <class Error>
-    struct _virt_completion<set_error_t(Error)>
+    template <class CPO, class... Args>
+    struct _virt_completion<CPO(Args...)>
     {
       _virt_completion() = default;
 
       _virt_completion(_virt_completion&&) = delete;
 
-      virtual void set_error(Error&& err) noexcept = 0;
-
-     protected:
-      ~_virt_completion() = default;
-    };
-
-    template <>
-    struct _virt_completion<set_stopped_t()>
-    {
-      _virt_completion() = default;
-
-      _virt_completion(_virt_completion&&) = delete;
-
-      virtual void set_stopped() noexcept = 0;
-
-     protected:
-      ~_virt_completion() = default;
-    };
-
-    template <class... Values>
-    struct _virt_completion<set_value_t(Values...)>
-    {
-      _virt_completion() = default;
-
-      _virt_completion(_virt_completion&&) = delete;
-
-      virtual void set_value(Values&&... values) noexcept = 0;
+      virtual void complete(CPO, Args&&...) noexcept = 0;
 
      protected:
       ~_virt_completion() = default;
@@ -96,55 +70,24 @@ namespace experimental::execution
     struct _virt_completions;
 
     template <class... Sigs>
-    struct _virt_completions<completion_signatures<Sigs...>> : virtual _virt_completion<Sigs>...
+    struct _virt_completions<completion_signatures<Sigs...>> : _virt_completion<Sigs>...
     {
       _virt_completions() = default;
 
       _virt_completions(_virt_completions&&) = delete;
 
+      using _virt_completion<Sigs>::complete...;
+
      protected:
       ~_virt_completions() = default;
     };
 
-    template <class Sig, class Derived>
-    struct _func_rcvr_base;
-
-    template <class Error, class Derived>
-    struct _func_rcvr_base<set_error_t(Error), Derived>
-    {
-      void set_error(Error&& err) && noexcept
-      {
-        static_cast<Derived*>(this)->completer_->set_error(std::forward<Error>(err));
-      }
-    };
-
-    template <class Derived>
-    struct _func_rcvr_base<set_stopped_t(), Derived>
-    {
-      void set_stopped() && noexcept
-      {
-        static_cast<Derived*>(this)->completer_->set_stopped();
-      }
-    };
-
-    template <class... Value, class Derived>
-    struct _func_rcvr_base<set_value_t(Value...), Derived>
-    {
-      void set_value(Value&&... value) && noexcept
-      {
-        static_cast<Derived*>(this)->completer_->set_value(std::forward<Value>(value)...);
-      }
-    };
-
     template <class Sigs>
     class _func_rcvr;
 
     template <class... Sigs>
     class _func_rcvr<completion_signatures<Sigs...>>
-      : public _func_rcvr_base<Sigs, _func_rcvr<completion_signatures<Sigs...>>>...
     {
-      friend _func_rcvr_base<Sigs, _func_rcvr>...;
-
       using completer_t = _virt_completions<completion_signatures<Sigs...>>;
 
       completer_t* completer_;
@@ -156,6 +99,28 @@ namespace experimental::execution
         : completer_(std::addressof(completer))
       {}
 
+      template <class Error>
+      void set_error(Error&& err) && noexcept
+        requires requires { this->completer_->complete(set_error_t{}, std::forward<Error>(err)); }
+      {
+        this->completer_->complete(set_error_t{}, std::forward<Error>(err));
+      }
+
+      void set_stopped() && noexcept
+        requires requires { this->completer_->complete(set_stopped_t{}); }
+      {
+        this->completer_->complete(set_stopped_t{});
+      }
+
+      template <class... Values>
+      void set_value(Values&&... values) && noexcept
+        requires requires {
+          this->completer_->complete(set_value_t{}, std::forward<Values>(values)...);
+        }
+      {
+        this->completer_->complete(set_value_t{}, std::forward<Values>(values)...);
+      }
+
       // TODO: get_env
     };
 
@@ -180,9 +145,9 @@ namespace experimental::execution
 
       _derived_op(_derived_op&&) = delete;
 
-      ~_derived_op() override = default;
+      ~_derived_op() final = default;
 
-      void start() & noexcept override
+      void start() & noexcept final
       {
         ::STDEXEC::start(op_);
       }
@@ -191,35 +156,20 @@ namespace experimental::execution
       connect_result_t<Sender, Receiver> op_;
     };
 
-    template <class Sig, class Derived>
+    template <class Base, class Derived, class... Sigs>
     struct _func_op_completion;
 
-    template <class Error, class Derived>
-    struct _func_op_completion<set_error_t(Error), Derived>
-      : virtual _virt_completion<set_error_t(Error)>
-    {
-      void set_error(Error&& err) noexcept final
-      {
-        static_cast<Derived*>(this)->complete(set_error_t{}, std::forward<Error>(err));
-      }
-    };
-
-    template <class Derived>
-    struct _func_op_completion<set_stopped_t(), Derived> : virtual _virt_completion<set_stopped_t()>
-    {
-      void set_stopped() noexcept final
-      {
-        static_cast<Derived*>(this)->complete(set_stopped_t{});
-      }
-    };
+    template <class Base, class Derived>
+    struct _func_op_completion<Base, Derived> : Base
+    {};
 
-    template <class... Value, class Derived>
-    struct _func_op_completion<set_value_t(Value...), Derived>
-      : virtual _virt_completion<set_value_t(Value...)>
+    template <class Base, class Derived, class CPO, class... Args, class... Sigs>
+    struct _func_op_completion<Base, Derived, CPO(Args...), Sigs...>
+      : _func_op_completion<Base, Derived, Sigs...>
     {
-      void set_value(Value&&... value) noexcept final
+      void complete(CPO, Args&&... args) noexcept final
       {
-        static_cast<Derived*>(this)->complete(set_value_t{}, std::forward<Value>(value)...);
+        static_cast<Derived*>(this)->complete(CPO{}, std::forward<Args>(args)...);
       }
     };
 
@@ -228,15 +178,17 @@ namespace experimental::execution
 
     template <class Receiver, class... Sigs>
     class _func_op<Receiver, completion_signatures<Sigs...>>
-      : private _virt_completions<completion_signatures<Sigs...>>
-      , private _func_op_completion<Sigs, _func_op<Receiver, completion_signatures<Sigs...>>>...
+      : private _func_op_completion<_virt_completions<completion_signatures<Sigs...>>,
+                                    _func_op<Receiver, completion_signatures<Sigs...>>,
+                                    Sigs...>
     {
       // TODO: use get_frame_allocator(get_env(rcvr_)) to allocate and destroy this
       std::unique_ptr<_base_op> op_;
       [[no_unique_address]]
       Receiver rcvr_;
 
-      friend _func_op_completion<Sigs, _func_op>...;
+      template <class B, class D, class... S>
+      friend struct _func_op_completion;
 
       template <class CPO, class... Arg>
       void complete(CPO cpo, Arg&&... arg) noexcept
@@ -279,8 +231,8 @@ namespace experimental::execution
         requires STDEXEC::__not_decays_to<Factory, _func_impl>  //
               && std::constructible_from<Factory>               //
               && STDEXEC::__callable<Factory, Args...>
-      //&& STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
-      //_func_rcvr<completion_signatures<Sigs...>>>
+              && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
+                                    _func_rcvr<completion_signatures<Sigs...>>>
       explicit(sizeof...(Args) == 0) _func_impl(Args&&... args, Factory&& factory)
         noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
         : args_(std::forward<Args>(args)...)
diff --git a/test/exec/test_function.cpp b/test/exec/test_function.cpp
index b855b38c0..3051ca452 100644
--- a/test/exec/test_function.cpp
+++ b/test/exec/test_function.cpp
@@ -25,7 +25,6 @@ namespace ex = STDEXEC;
 
 namespace
 {
-
   TEST_CASE("exec::function is constructible", "[types][function]")
   {
     exec::function<void()> voidSndr([]() noexcept { return ex::just(); });
@@ -49,10 +48,22 @@ namespace
 
   TEST_CASE("exec::function is connectable", "[types][function]")
   {
-    exec::function<int()> sndr([]() noexcept { return ex::just(42); });
+    exec::function<int() noexcept> sndr([]() noexcept { return ex::just(42); });
 
-    auto [fortytwo] = ex::sync_wait(std::move(sndr)).value();
+    struct rcvr
+    {
+      using receiver_concept = ex::receiver_tag;
+
+      void set_value(int) && noexcept {}
+      void set_stopped() && noexcept {}
+    };
 
+    STATIC_REQUIRE(ex::receiver<rcvr>);
+
+    auto op = ex::connect(std::move(sndr), rcvr{});
+
+    auto [fortytwo] = ex::sync_wait(std::move(sndr)).value();
+    
     REQUIRE(fortytwo == 42);
   }
 }  // namespace

From de5ccbb456e1d03a77b7043ee4110d2d91b108f7 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sun, 19 Apr 2026 08:03:14 -0700
Subject: [PATCH 06/20] Support arbitrary completion signatures

`function<ex::sender_tag(Args...), ex::completion_signatures<...>>` now
declares an async function mapping `Args...` to the explicitly specified
completion signatures.
---
 include/exec/function.hpp   | 19 ++++++++++++++-----
 test/exec/test_function.cpp |  9 ++++++++-
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 9a62f4dc2..901e589dc 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -309,13 +309,13 @@ namespace experimental::execution
 
   // TODO: think about environment forwarding
   template <class...>
-  class function;
+  struct function;
 
   template <class Return, class... Args, bool NoThrow>
-  class function<Return(Args...) noexcept(NoThrow)>
-    : public _func::_func_impl<STDEXEC::sender_tag(Args...),
-                               _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
-                               STDEXEC::env<>>
+  struct function<Return(Args...) noexcept(NoThrow)>
+    : _func::_func_impl<STDEXEC::sender_tag(Args...),
+                        _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
+                        STDEXEC::env<>>
   {
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
                                    _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
@@ -323,6 +323,15 @@ namespace experimental::execution
 
     using base::base;
   };
+
+  template <class... Args, STDEXEC::__is_instance_of<STDEXEC::completion_signatures> Sigs>
+  struct function<STDEXEC::sender_tag(Args...), Sigs>
+    : _func::_func_impl<STDEXEC::sender_tag(Args...), Sigs, STDEXEC::env<>>
+  {
+    using base = _func::_func_impl<STDEXEC::sender_tag(Args...), Sigs, STDEXEC::env<>>;
+
+    using base::base;
+  };
 }  // namespace experimental::execution
 
 namespace exec = experimental::execution;
diff --git a/test/exec/test_function.cpp b/test/exec/test_function.cpp
index 3051ca452..146705588 100644
--- a/test/exec/test_function.cpp
+++ b/test/exec/test_function.cpp
@@ -39,11 +39,18 @@ namespace
     exec::function<void() noexcept> nothrowSndr([]() noexcept { return ex::just(); });
     exec::function<int() noexcept>  nothrowIntSndr([]() noexcept { return ex::just(42); });
 
+    exec::function<ex::sender_tag(), ex::completion_signatures<ex::set_value_t(int)>> unstoppable(
+      []() noexcept { return ex::just(42); });
+    exec::function<ex::sender_tag(), ex::completion_signatures<ex::set_stopped_t()>> onlystopped(
+      []() noexcept { return ex::just_stopped(); });
+
     STATIC_REQUIRE(STDEXEC::sender<decltype(voidSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(intSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(binarySndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(nothrowSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(nothrowIntSndr)>);
+    STATIC_REQUIRE(STDEXEC::sender<decltype(unstoppable)>);
+    STATIC_REQUIRE(STDEXEC::sender<decltype(onlystopped)>);
   }
 
   TEST_CASE("exec::function is connectable", "[types][function]")
@@ -63,7 +70,7 @@ namespace
     auto op = ex::connect(std::move(sndr), rcvr{});
 
     auto [fortytwo] = ex::sync_wait(std::move(sndr)).value();
-    
+
     REQUIRE(fortytwo == 42);
   }
 }  // namespace

From 18d10f10d93f98c1af8e8c9af2d3a5036c110a91 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sun, 19 Apr 2026 08:19:20 -0700
Subject: [PATCH 07/20] Round out the partial specializations of exec::function

Support for explicit completion signatures, environment, or both in the
declaration of an `exec:function`.
---
 include/exec/function.hpp   | 35 ++++++++++++++++++++++++++++++++++-
 test/exec/test_function.cpp |  7 +++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 901e589dc..21e58ba02 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -312,6 +312,13 @@ namespace experimental::execution
   struct function;
 
   template <class Return, class... Args, bool NoThrow>
+  // should this require STDEXEC::__not_same_as<Return, STDEXEC::sender_tag>?
+  //
+  // you *could* write STDEXEC::just(STDEXEC::sender_tag{}), but it seems more likely
+  // that invokign this specialization with Return set to sender_tag is a bug...
+  //
+  // the same question applies to all the specializations below that take explicit
+  // completion signatures
   struct function<Return(Args...) noexcept(NoThrow)>
     : _func::_func_impl<STDEXEC::sender_tag(Args...),
                         _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
@@ -324,7 +331,8 @@ namespace experimental::execution
     using base::base;
   };
 
-  template <class... Args, STDEXEC::__is_instance_of<STDEXEC::completion_signatures> Sigs>
+  template <class... Args, class Sigs>
+    requires STDEXEC::__is_instance_of<Sigs, STDEXEC::completion_signatures>
   struct function<STDEXEC::sender_tag(Args...), Sigs>
     : _func::_func_impl<STDEXEC::sender_tag(Args...), Sigs, STDEXEC::env<>>
   {
@@ -332,6 +340,31 @@ namespace experimental::execution
 
     using base::base;
   };
+
+  template <class Return, class... Args, bool NoThrow, class Env>
+    requires STDEXEC::__is_not_instance_of<Env, STDEXEC::completion_signatures>
+  struct function<Return(Args...) noexcept(NoThrow), Env>
+    : _func::_func_impl<STDEXEC::sender_tag(Args...),
+                        _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
+                        Env>
+  {
+    using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
+                                   _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
+                                   Env>;
+
+    using base::base;
+  };
+
+  template <class... Args, class... Sigs, class Env>
+    requires STDEXEC::__is_not_instance_of<Env, STDEXEC::completion_signatures>
+  struct function<STDEXEC::sender_tag(Args...), STDEXEC::completion_signatures<Sigs...>, Env>
+    : _func::_func_impl<STDEXEC::sender_tag(Args...), STDEXEC::completion_signatures<Sigs...>, Env>
+  {
+    using base =
+      _func::_func_impl<STDEXEC::sender_tag(Args...), STDEXEC::completion_signatures<Sigs...>, Env>;
+
+    using base::base;
+  };
 }  // namespace experimental::execution
 
 namespace exec = experimental::execution;
diff --git a/test/exec/test_function.cpp b/test/exec/test_function.cpp
index 146705588..bf3f5dc0a 100644
--- a/test/exec/test_function.cpp
+++ b/test/exec/test_function.cpp
@@ -44,6 +44,11 @@ namespace
     exec::function<ex::sender_tag(), ex::completion_signatures<ex::set_stopped_t()>> onlystopped(
       []() noexcept { return ex::just_stopped(); });
 
+    exec::function<void(), ex::env<>> trivialCustomEnv([]() noexcept { return ex::just(); });
+
+    exec::function<ex::sender_tag(int), ex::completion_signatures<ex::set_value_t()>, ex::env<>>
+      totalControl(5, [](int) noexcept { return ex::just(); });
+
     STATIC_REQUIRE(STDEXEC::sender<decltype(voidSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(intSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(binarySndr)>);
@@ -51,6 +56,8 @@ namespace
     STATIC_REQUIRE(STDEXEC::sender<decltype(nothrowIntSndr)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(unstoppable)>);
     STATIC_REQUIRE(STDEXEC::sender<decltype(onlystopped)>);
+    STATIC_REQUIRE(STDEXEC::sender<decltype(trivialCustomEnv)>);
+    STATIC_REQUIRE(STDEXEC::sender<decltype(totalControl)>);
   }
 
   TEST_CASE("exec::function is connectable", "[types][function]")

From 846b53e69ec973e3a53fa62389bd3691a0093511 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sun, 19 Apr 2026 08:23:03 -0700
Subject: [PATCH 08/20] Delete a layer of forwarding

---
 include/exec/function.hpp | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 21e58ba02..d23308baa 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -169,7 +169,8 @@ namespace experimental::execution
     {
       void complete(CPO, Args&&... args) noexcept final
       {
-        static_cast<Derived*>(this)->complete(CPO{}, std::forward<Args>(args)...);
+        auto& rcvr = static_cast<Derived*>(this)->rcvr_;
+        CPO{}(std::move(rcvr), std::forward<Args>(args)...);
       }
     };
 
@@ -190,12 +191,6 @@ namespace experimental::execution
       template <class B, class D, class... S>
       friend struct _func_op_completion;
 
-      template <class CPO, class... Arg>
-      void complete(CPO cpo, Arg&&... arg) noexcept
-      {
-        std::move(cpo)(std::move(rcvr_), std::forward<Arg>(arg)...);
-      }
-
      public:
       using operation_state_concept = operation_state_tag;
 

From 320a92369fce3320a6e773cf2fb16934476e6c69 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sun, 19 Apr 2026 12:50:23 -0700
Subject: [PATCH 09/20] Inch towards allocator support

Rework the dynamically allocated operation state type to support
allocators, but always use `std::allocator` for now.
---
 include/exec/function.hpp | 43 +++++++++++++++++++++++++++++++++------
 1 file changed, 37 insertions(+), 6 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index d23308baa..f7c16194e 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -19,10 +19,12 @@
 #include "../stdexec/__detail/__concepts.hpp"
 #include "../stdexec/__detail/__env.hpp"
 #include "../stdexec/__detail/__receivers.hpp"
+#include "../stdexec/__detail/__scope.hpp"
 #include "../stdexec/__detail/__sender_concepts.hpp"
 
 #include <exception>
 #include <memory>
+#include <new>
 #include <tuple>
 #include <type_traits>
 #include <utility>
@@ -135,12 +137,13 @@ namespace experimental::execution
       virtual void start() & noexcept = 0;
     };
 
-    template <class Sender, class Receiver>
+    template <class Sender, class Receiver, class Allocator>
     struct _derived_op : _base_op
     {
-      explicit _derived_op(Sender&& sndr, Receiver rcvr)
+      explicit _derived_op(Sender&& sndr, Receiver rcvr, Allocator const & alloc)
         noexcept(std::is_nothrow_invocable_v<connect_t, Sender, Receiver>)
         : op_(connect(std::forward<Sender>(sndr), std::move(rcvr)))
+        , alloc_(alloc)
       {}
 
       _derived_op(_derived_op&&) = delete;
@@ -152,8 +155,19 @@ namespace experimental::execution
         ::STDEXEC::start(op_);
       }
 
+      static constexpr void operator delete(_derived_op* p, std::destroying_delete_t)
+      {
+        using traits = std::allocator_traits<Allocator>::template rebind_traits<_derived_op>;
+
+        typename traits::allocator_type alloc = std::move(p->alloc_);
+        traits::destroy(alloc, p);
+        traits::deallocate(alloc, p, 1);
+      }
+
      private:
       connect_result_t<Sender, Receiver> op_;
+      [[no_unique_address]]
+      Allocator alloc_;
     };
 
     template <class Base, class Derived, class... Sigs>
@@ -215,7 +229,7 @@ namespace experimental::execution
     template <class SndrCncpt, class... Args, class... Sigs, class Env>
     class _func_impl<SndrCncpt(Args...), completion_signatures<Sigs...>, Env>
     {
-      _base_op* (*factory_)(_func_rcvr<completion_signatures<Sigs...>>, Args&&...);
+      std::unique_ptr<_base_op> (*factory_)(_func_rcvr<completion_signatures<Sigs...>>, Args&&...);
       [[no_unique_address]]
       std::tuple<Args...> args_;
 
@@ -235,13 +249,30 @@ namespace experimental::execution
         using sender_t   = std::invoke_result_t<Factory, Args...>;
         using receiver_t = _func_rcvr<completion_signatures<Sigs...>>;
 
-        using op_t = _derived_op<sender_t, receiver_t>;
+        using op_t = _derived_op<sender_t, receiver_t, std::allocator<std::byte>>;
 
-        factory_ = [](receiver_t rcvr, Args&&... args) -> _base_op*
+        factory_ = [](receiver_t rcvr, Args&&... args) -> std::unique_ptr<_base_op>
         {
+          using traits = std::allocator_traits<std::allocator<op_t>>;
+
           Factory factory;
+
           // TODO: query rcvr for a frame allocator and use it
-          return new op_t(factory(std::forward<Args>(args)...), std::move(rcvr));
+          typename traits::allocator_type alloc;
+
+          auto* op = traits::allocate(alloc, 1);
+
+          __scope_guard guard{[&]() noexcept { traits::deallocate(alloc, op, 1); }};
+
+          traits::construct(alloc,
+                            op,
+                            factory(std::forward<Args>(args)...),
+                            std::move(rcvr),
+                            alloc);
+
+          guard.__dismiss();
+
+          return std::unique_ptr<_base_op>(op);
         };
       }
 

From 8b72f780e2ff88595bebc167cc736b5794c12e6c Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sun, 19 Apr 2026 14:02:31 -0700
Subject: [PATCH 10/20] Add frame allocator support

This diff needs tests, but the existing tests build and pass, which
seems like a good signal. I've added a `get_frame_allocator` query, and
a defaulting cascade from `get_frame_allocator` -> `get_allocator` ->
`std::allocator<std::byte>` to the allocation of `_derived_op`.
---
 include/exec/function.hpp | 50 +++++++++++++++++++++++++++++++++++----
 1 file changed, 46 insertions(+), 4 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index f7c16194e..16d078dfb 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -18,6 +18,7 @@
 #include "../stdexec/__detail/__completion_signatures.hpp"
 #include "../stdexec/__detail/__concepts.hpp"
 #include "../stdexec/__detail/__env.hpp"
+#include "../stdexec/__detail/__read_env.hpp"
 #include "../stdexec/__detail/__receivers.hpp"
 #include "../stdexec/__detail/__scope.hpp"
 #include "../stdexec/__detail/__sender_concepts.hpp"
@@ -48,6 +49,31 @@
 // the frame allocator from the environment without relying on TLS.
 namespace experimental::execution
 {
+  struct get_frame_allocator_t : STDEXEC::__query<get_frame_allocator_t>
+  {
+    using STDEXEC::__query<get_frame_allocator_t>::operator();
+
+    constexpr auto operator()() const noexcept
+    {
+      return STDEXEC::read_env(get_frame_allocator_t{});
+    }
+
+    template <class Env>
+    static constexpr void __validate() noexcept
+    {
+      static_assert(STDEXEC::__nothrow_callable<get_frame_allocator_t, Env const &>);
+      using __alloc_t = STDEXEC::__call_result_t<get_frame_allocator_t, Env const &>;
+      static_assert(STDEXEC::__simple_allocator<STDEXEC::__decay_t<__alloc_t>>);
+    }
+
+    static consteval auto query(STDEXEC::forwarding_query_t) noexcept -> bool
+    {
+      return true;
+    }
+  };
+
+  inline constexpr get_frame_allocator_t get_frame_allocator{};
+
   namespace _func
   {
     using namespace STDEXEC;
@@ -197,7 +223,6 @@ namespace experimental::execution
                                     _func_op<Receiver, completion_signatures<Sigs...>>,
                                     Sigs...>
     {
-      // TODO: use get_frame_allocator(get_env(rcvr_)) to allocate and destroy this
       std::unique_ptr<_base_op> op_;
       [[no_unique_address]]
       Receiver rcvr_;
@@ -223,6 +248,23 @@ namespace experimental::execution
       }
     };
 
+    template <class Env>
+    constexpr auto choose_frame_allocator(Env const & env) noexcept
+    {
+      if constexpr (requires { get_frame_allocator(env); })
+      {
+        return get_frame_allocator(env);
+      }
+      else if constexpr (requires { get_allocator(env); })
+      {
+        return get_allocator(env);
+      }
+      else
+      {
+        return std::allocator<std::byte>();
+      }
+    }
+
     template <class Args, class Sigs, class Env>
     class _func_impl;
 
@@ -253,12 +295,12 @@ namespace experimental::execution
 
         factory_ = [](receiver_t rcvr, Args&&... args) -> std::unique_ptr<_base_op>
         {
-          using traits = std::allocator_traits<std::allocator<op_t>>;
+          using traits = std::allocator_traits<decltype(choose_frame_allocator(
+            get_env(rcvr)))>::template rebind_traits<op_t>;
 
           Factory factory;
 
-          // TODO: query rcvr for a frame allocator and use it
-          typename traits::allocator_type alloc;
+          typename traits::allocator_type alloc(choose_frame_allocator(get_env(rcvr)));
 
           auto* op = traits::allocate(alloc, 1);
 

From 93b5c5ec530a9e1eb6681706be8c9a707edaacd8 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sun, 19 Apr 2026 17:26:17 -0700
Subject: [PATCH 11/20] constexpr (almost) all the things

This diff marks almost every function `constexpr`. It doesn't mark the
imlementation of `complete` in the CRTP `_func_op_completion` class
template because Clang rejects the down-cast to `Derived` as not a
core constant expression; apparently, `Derived` is incomplete when it's
being evaluated as a side effect of constraint satisfaction testing.

This `constexpr` "hole" means `exec::function` can't be used at compile
time, but maybe it can be worked around later.
---
 include/exec/function.hpp | 45 ++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 19 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 16d078dfb..0a3503731 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -22,6 +22,7 @@
 #include "../stdexec/__detail/__receivers.hpp"
 #include "../stdexec/__detail/__scope.hpp"
 #include "../stdexec/__detail/__sender_concepts.hpp"
+#include "../stdexec/execution.hpp"
 
 #include <exception>
 #include <memory>
@@ -84,14 +85,14 @@ namespace experimental::execution
     template <class CPO, class... Args>
     struct _virt_completion<CPO(Args...)>
     {
-      _virt_completion() = default;
+      constexpr _virt_completion() = default;
 
       _virt_completion(_virt_completion&&) = delete;
 
-      virtual void complete(CPO, Args&&...) noexcept = 0;
+      constexpr virtual void complete(CPO, Args&&...) noexcept = 0;
 
      protected:
-      ~_virt_completion() = default;
+      constexpr ~_virt_completion() = default;
     };
 
     template <class Sigs>
@@ -100,14 +101,14 @@ namespace experimental::execution
     template <class... Sigs>
     struct _virt_completions<completion_signatures<Sigs...>> : _virt_completion<Sigs>...
     {
-      _virt_completions() = default;
+      constexpr _virt_completions() = default;
 
       _virt_completions(_virt_completions&&) = delete;
 
       using _virt_completion<Sigs>::complete...;
 
      protected:
-      ~_virt_completions() = default;
+      constexpr ~_virt_completions() = default;
     };
 
     template <class Sigs>
@@ -123,25 +124,25 @@ namespace experimental::execution
      public:
       using receiver_concept = receiver_tag;
 
-      explicit _func_rcvr(completer_t& completer) noexcept
+      constexpr explicit _func_rcvr(completer_t& completer) noexcept
         : completer_(std::addressof(completer))
       {}
 
       template <class Error>
-      void set_error(Error&& err) && noexcept
+      constexpr void set_error(Error&& err) && noexcept
         requires requires { this->completer_->complete(set_error_t{}, std::forward<Error>(err)); }
       {
         this->completer_->complete(set_error_t{}, std::forward<Error>(err));
       }
 
-      void set_stopped() && noexcept
+      constexpr void set_stopped() && noexcept
         requires requires { this->completer_->complete(set_stopped_t{}); }
       {
         this->completer_->complete(set_stopped_t{});
       }
 
       template <class... Values>
-      void set_value(Values&&... values) && noexcept
+      constexpr void set_value(Values&&... values) && noexcept
         requires requires {
           this->completer_->complete(set_value_t{}, std::forward<Values>(values)...);
         }
@@ -154,19 +155,19 @@ namespace experimental::execution
 
     struct _base_op
     {
-      _base_op() = default;
+      constexpr _base_op() = default;
 
       _base_op(_base_op&&) = delete;
 
-      virtual ~_base_op() = default;
+      constexpr virtual ~_base_op() = default;
 
-      virtual void start() & noexcept = 0;
+      constexpr virtual void start() & noexcept = 0;
     };
 
     template <class Sender, class Receiver, class Allocator>
     struct _derived_op : _base_op
     {
-      explicit _derived_op(Sender&& sndr, Receiver rcvr, Allocator const & alloc)
+      constexpr explicit _derived_op(Sender&& sndr, Receiver rcvr, Allocator const & alloc)
         noexcept(std::is_nothrow_invocable_v<connect_t, Sender, Receiver>)
         : op_(connect(std::forward<Sender>(sndr), std::move(rcvr)))
         , alloc_(alloc)
@@ -174,9 +175,9 @@ namespace experimental::execution
 
       _derived_op(_derived_op&&) = delete;
 
-      ~_derived_op() final = default;
+      constexpr ~_derived_op() final = default;
 
-      void start() & noexcept final
+      constexpr void start() & noexcept final
       {
         ::STDEXEC::start(op_);
       }
@@ -209,6 +210,12 @@ namespace experimental::execution
     {
       void complete(CPO, Args&&... args) noexcept final
       {
+        // This seems like it ought to be true, but it fails...
+        //
+        // Some testing shows it's being evaluated when Derive is incomplete
+	// during constraint satisfaction testing.
+        //
+        // static_assert(std::derived_from<_func_op_completion, Derived>);
         auto& rcvr = static_cast<Derived*>(this)->rcvr_;
         CPO{}(std::move(rcvr), std::forward<Args>(args)...);
       }
@@ -234,15 +241,15 @@ namespace experimental::execution
       using operation_state_concept = operation_state_tag;
 
       template <class Factory>
-      _func_op(Receiver rcvr, Factory factory)
+      constexpr _func_op(Receiver rcvr, Factory factory)
         : rcvr_(std::move(rcvr))
         , op_(factory(_func_rcvr<completion_signatures<Sigs...>>(*this))){};
 
       _func_op(_func_op&&) = delete;
 
-      ~_func_op() = default;
+      constexpr ~_func_op() = default;
 
-      void start() & noexcept
+      constexpr void start() & noexcept
       {
         op_->start();
       }
@@ -284,7 +291,7 @@ namespace experimental::execution
               && STDEXEC::__callable<Factory, Args...>
               && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
                                     _func_rcvr<completion_signatures<Sigs...>>>
-      explicit(sizeof...(Args) == 0) _func_impl(Args&&... args, Factory&& factory)
+      constexpr explicit(sizeof...(Args) == 0) _func_impl(Args&&... args, Factory&& factory)
         noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
         : args_(std::forward<Args>(args)...)
       {

From 83e7ed785934786148156931c3495a6a9441a42b Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Sun, 19 Apr 2026 18:35:26 -0700
Subject: [PATCH 12/20] More tests

Validate that more kinds of senders can be erased and then connected and
started. Also clean up the captures in some lambdas in `connect` and
`clang-format`.
---
 include/exec/function.hpp   |  6 +++---
 test/exec/test_function.cpp | 39 +++++++++++++++++++++++++++----------
 2 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 0a3503731..531bc0960 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -213,7 +213,7 @@ namespace experimental::execution
         // This seems like it ought to be true, but it fails...
         //
         // Some testing shows it's being evaluated when Derive is incomplete
-	// during constraint satisfaction testing.
+        // during constraint satisfaction testing.
         //
         // static_assert(std::derived_from<_func_op_completion, Derived>);
         auto& rcvr = static_cast<Derived*>(this)->rcvr_;
@@ -336,10 +336,10 @@ namespace experimental::execution
       constexpr _func_op<Receiver, completion_signatures<Sigs...>> connect(Receiver rcvr)
       {
         return {std::move(rcvr),
-                [&, this](auto rcvr)
+                [this](auto rcvr)
                 {
                   return std::apply(
-                    [&](Args&&... args)
+                    [&rcvr, this](Args&&... args)
                     { return factory_(std::move(rcvr), std::forward<Args>(args)...); },
                     std::move(args_));
                 }};
diff --git a/test/exec/test_function.cpp b/test/exec/test_function.cpp
index bf3f5dc0a..1a2d5aefe 100644
--- a/test/exec/test_function.cpp
+++ b/test/exec/test_function.cpp
@@ -62,22 +62,41 @@ namespace
 
   TEST_CASE("exec::function is connectable", "[types][function]")
   {
-    exec::function<int() noexcept> sndr([]() noexcept { return ex::just(42); });
+    {
+      exec::function<int() noexcept> sndr([]() noexcept { return ex::just(42); });
+
+      auto [fortytwo] = ex::sync_wait(std::move(sndr)).value();
+
+      REQUIRE(fortytwo == 42);
+    }
 
-    struct rcvr
     {
-      using receiver_concept = ex::receiver_tag;
+      exec::function<void()> sndr([]() -> decltype(ex::just()) { throw "oops"; });
 
-      void set_value(int) && noexcept {}
-      void set_stopped() && noexcept {}
-    };
+      REQUIRE_THROWS(ex::sync_wait(std::move(sndr)));
+    }
 
-    STATIC_REQUIRE(ex::receiver<rcvr>);
+    {
+      exec::function<void()> sndr([]() noexcept
+                                  { return ex::just() | ex::then([] { throw "oops"; }); });
 
-    auto op = ex::connect(std::move(sndr), rcvr{});
+      REQUIRE_THROWS(ex::sync_wait(std::move(sndr)));
+    }
 
-    auto [fortytwo] = ex::sync_wait(std::move(sndr)).value();
+    {
+      exec::function<void()> sndr([]() noexcept { return ex::just_stopped(); });
+
+      auto ret = ex::sync_wait(std::move(sndr));
+
+      REQUIRE_FALSE(ret.has_value());
+    }
+
+    {
+      exec::function<ex::sender_tag(),
+                     ex::completion_signatures<ex::set_value_t(), ex::set_error_t(int)>>
+        sndr([]() noexcept { return ex::just_error(42); });
 
-    REQUIRE(fortytwo == 42);
+      REQUIRE_THROWS_AS(ex::sync_wait(std::move(sndr)), int);
+    }
   }
 }  // namespace

From 65acee431f21ce381c23090f6cbcfad542fe6edd Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Mon, 20 Apr 2026 07:40:51 -0700
Subject: [PATCH 13/20] Get allocator selection working

Still TODO is that the `get_frame_allocator` query shouldn't have to be
specified in the `function`'s custom environment (and, come to think of
it, neither should `get_allocator`), but, when specified, it works.
---
 include/exec/function.hpp   | 75 +++++++++++++++++++++++++------------
 test/exec/test_function.cpp | 28 ++++++++++++++
 2 files changed, 80 insertions(+), 23 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 531bc0960..c965423d1 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -95,11 +95,11 @@ namespace experimental::execution
       constexpr ~_virt_completion() = default;
     };
 
-    template <class Sigs>
+    template <class Sigs, class Env>
     struct _virt_completions;
 
-    template <class... Sigs>
-    struct _virt_completions<completion_signatures<Sigs...>> : _virt_completion<Sigs>...
+    template <class... Sigs, class Env>
+    struct _virt_completions<completion_signatures<Sigs...>, Env> : _virt_completion<Sigs>...
     {
       constexpr _virt_completions() = default;
 
@@ -107,17 +107,19 @@ namespace experimental::execution
 
       using _virt_completion<Sigs>::complete...;
 
+      virtual Env get_env() const noexcept = 0;
+
      protected:
       constexpr ~_virt_completions() = default;
     };
 
-    template <class Sigs>
+    template <class Sigs, class Env>
     class _func_rcvr;
 
-    template <class... Sigs>
-    class _func_rcvr<completion_signatures<Sigs...>>
+    template <class... Sigs, class Env>
+    class _func_rcvr<completion_signatures<Sigs...>, Env>
     {
-      using completer_t = _virt_completions<completion_signatures<Sigs...>>;
+      using completer_t = _virt_completions<completion_signatures<Sigs...>, Env>;
 
       completer_t* completer_;
 
@@ -150,7 +152,10 @@ namespace experimental::execution
         this->completer_->complete(set_value_t{}, std::forward<Values>(values)...);
       }
 
-      // TODO: get_env
+      constexpr auto get_env() const noexcept -> env_of_t<completer_t>
+      {
+        return STDEXEC::get_env(*completer_);
+      }
     };
 
     struct _base_op
@@ -221,13 +226,13 @@ namespace experimental::execution
       }
     };
 
-    template <class Receiver, class Sigs>
+    template <class Receiver, class Sigs, class Env>
     class _func_op;
 
-    template <class Receiver, class... Sigs>
-    class _func_op<Receiver, completion_signatures<Sigs...>>
-      : private _func_op_completion<_virt_completions<completion_signatures<Sigs...>>,
-                                    _func_op<Receiver, completion_signatures<Sigs...>>,
+    template <class Receiver, class... Sigs, class Env>
+    class _func_op<Receiver, completion_signatures<Sigs...>, Env>
+      : private _func_op_completion<_virt_completions<completion_signatures<Sigs...>, Env>,
+                                    _func_op<Receiver, completion_signatures<Sigs...>, Env>,
                                     Sigs...>
     {
       std::unique_ptr<_base_op> op_;
@@ -237,13 +242,27 @@ namespace experimental::execution
       template <class B, class D, class... S>
       friend struct _func_op_completion;
 
+      constexpr Env get_env() const noexcept final
+      {
+        using RcvrEnv = env_of_t<Receiver>;
+
+        if constexpr (std::constructible_from<Env, RcvrEnv>)
+        {
+          return Env(::STDEXEC::get_env(rcvr_));
+        }
+        else
+        {
+          return {};
+        }
+      }
+
      public:
       using operation_state_concept = operation_state_tag;
 
       template <class Factory>
       constexpr _func_op(Receiver rcvr, Factory factory)
         : rcvr_(std::move(rcvr))
-        , op_(factory(_func_rcvr<completion_signatures<Sigs...>>(*this))){};
+        , op_(factory(_func_rcvr<completion_signatures<Sigs...>, Env>(*this))){};
 
       _func_op(_func_op&&) = delete;
 
@@ -278,7 +297,8 @@ namespace experimental::execution
     template <class SndrCncpt, class... Args, class... Sigs, class Env>
     class _func_impl<SndrCncpt(Args...), completion_signatures<Sigs...>, Env>
     {
-      std::unique_ptr<_base_op> (*factory_)(_func_rcvr<completion_signatures<Sigs...>>, Args&&...);
+      std::unique_ptr<_base_op> (*factory_)(_func_rcvr<completion_signatures<Sigs...>, Env>,
+                                            Args&&...);
       [[no_unique_address]]
       std::tuple<Args...> args_;
 
@@ -290,29 +310,38 @@ namespace experimental::execution
               && std::constructible_from<Factory>               //
               && STDEXEC::__callable<Factory, Args...>
               && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
-                                    _func_rcvr<completion_signatures<Sigs...>>>
+                                    _func_rcvr<completion_signatures<Sigs...>, Env>>
       constexpr explicit(sizeof...(Args) == 0) _func_impl(Args&&... args, Factory&& factory)
         noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
         : args_(std::forward<Args>(args)...)
       {
         using sender_t   = std::invoke_result_t<Factory, Args...>;
-        using receiver_t = _func_rcvr<completion_signatures<Sigs...>>;
-
-        using op_t = _derived_op<sender_t, receiver_t, std::allocator<std::byte>>;
+        using receiver_t = _func_rcvr<completion_signatures<Sigs...>, Env>;
 
         factory_ = [](receiver_t rcvr, Args&&... args) -> std::unique_ptr<_base_op>
         {
-          using traits = std::allocator_traits<decltype(choose_frame_allocator(
-            get_env(rcvr)))>::template rebind_traits<op_t>;
+          // the type of the allocator provided by the receiver's environment
+          using alloc_t = decltype(choose_frame_allocator(get_env(rcvr)));
+          // the traits for that allocator, but normalized to std::byte to minimize
+          // template instantiations
+          using traits_t = std::allocator_traits<alloc_t>::template rebind_traits<std::byte>;
 
-          Factory factory;
+          // the type of operation we'll ultimately allocate, which depends on the type of
+          // the allocator we're using
+          using op_t = _derived_op<sender_t, receiver_t, typename traits_t::allocator_type>;
+
+          // finally, the allocator traits for an allocator that can allocate an op_t
+          using traits = traits_t::template rebind_traits<op_t>;
 
+	  // ...and the allocator itself
           typename traits::allocator_type alloc(choose_frame_allocator(get_env(rcvr)));
 
           auto* op = traits::allocate(alloc, 1);
 
           __scope_guard guard{[&]() noexcept { traits::deallocate(alloc, op, 1); }};
 
+          Factory factory;
+
           traits::construct(alloc,
                             op,
                             factory(std::forward<Args>(args)...),
@@ -333,7 +362,7 @@ namespace experimental::execution
       }
 
       template <class Receiver>
-      constexpr _func_op<Receiver, completion_signatures<Sigs...>> connect(Receiver rcvr)
+      constexpr _func_op<Receiver, completion_signatures<Sigs...>, Env> connect(Receiver rcvr)
       {
         return {std::move(rcvr),
                 [this](auto rcvr)
diff --git a/test/exec/test_function.cpp b/test/exec/test_function.cpp
index 1a2d5aefe..f927426b5 100644
--- a/test/exec/test_function.cpp
+++ b/test/exec/test_function.cpp
@@ -21,6 +21,8 @@
 
 #include <stdexec/execution.hpp>
 
+#include <memory>
+
 namespace ex = STDEXEC;
 
 namespace
@@ -99,4 +101,30 @@ namespace
       REQUIRE_THROWS_AS(ex::sync_wait(std::move(sndr)), int);
     }
   }
+
+  TEST_CASE("exec::function forwards get_frame_allocator", "[types][function]")
+  {
+    // TODO: you probably shouldn't have to specify the frame allocator query like this
+    using Env =
+      ex::env<ex::prop<exec::get_frame_allocator_t, std::pmr::polymorphic_allocator<std::byte>>>;
+
+    exec::function<bool() noexcept, Env> sndr(
+      []() noexcept
+      {
+        return ex::read_env(exec::get_frame_allocator)
+             | ex::then(
+                 [](auto alloc) noexcept
+                 {
+                   return std::same_as<std::pmr::polymorphic_allocator<std::byte>, decltype(alloc)>;
+                 });
+      });
+
+    std::pmr::polymorphic_allocator<std::byte> alloc;
+
+    auto [ret] = ex::sync_wait(std::move(sndr)
+                               | ex::write_env(ex::prop(exec::get_frame_allocator, alloc)))
+                   .value();
+
+    REQUIRE(ret);
+  }
 }  // namespace

From a738dd71d46a556a2fef422e6ff2dcd1827bd149 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Tue, 21 Apr 2026 15:33:55 -0700
Subject: [PATCH 14/20] Environment forwarding works(ish)

This needs cleaning up and a *lot* more tests, but the current tests
build and pass with a synthesized polymorphic environment.
---
 include/exec/function.hpp   | 336 +++++++++++++++++++++++++++---------
 test/exec/test_function.cpp |  14 +-
 2 files changed, 266 insertions(+), 84 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index c965423d1..28fda5db2 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -22,7 +22,6 @@
 #include "../stdexec/__detail/__receivers.hpp"
 #include "../stdexec/__detail/__scope.hpp"
 #include "../stdexec/__detail/__sender_concepts.hpp"
-#include "../stdexec/execution.hpp"
 
 #include <exception>
 #include <memory>
@@ -75,6 +74,41 @@ namespace experimental::execution
 
   inline constexpr get_frame_allocator_t get_frame_allocator{};
 
+  namespace _qry_detail
+  {
+    using namespace STDEXEC;
+
+    template <bool NoThrow, class Env, class Query, class... Args>
+    concept _conditionally_nothrow_queryable_with =
+      (!NoThrow && __queryable_with<Env const &, Query, Args...>)
+      || __nothrow_queryable_with<Env const &, Query, Args...>;
+
+    template <class Expected, bool NoThrow, class Env, class Query, class... Args>
+    concept _query_result_convertible_to =
+      (!NoThrow && std::is_convertible_v<__query_result_t<Env const &, Query, Args...>, Expected>)
+      || std::is_nothrow_convertible_v<__query_result_t<Env const &, Query, Args...>, Expected>;
+
+    template <class Sig>
+    struct query;
+
+    template <class Return, class Query, class... Args, bool NoThrow>
+    struct query<Return(Query, Args...) noexcept(NoThrow)>
+    {
+     protected:
+      template <class Env>
+        requires _conditionally_nothrow_queryable_with<NoThrow, Env, Query, Args...>
+              && _query_result_convertible_to<Return, NoThrow, Env, Query, Args...>
+      static Return query_delegate(Env const &env, Query query, Args &&...args) noexcept(NoThrow)
+      {
+        return __query<Query>()(env, std::forward<Args>(args)...);
+      }
+    };
+  }  // namespace _qry_detail
+
+  template <class... Queries>
+  struct queries : _qry_detail::query<Queries>...
+  {};
+
   namespace _func
   {
     using namespace STDEXEC;
@@ -87,51 +121,158 @@ namespace experimental::execution
     {
       constexpr _virt_completion() = default;
 
-      _virt_completion(_virt_completion&&) = delete;
+      _virt_completion(_virt_completion &&) = delete;
 
-      constexpr virtual void complete(CPO, Args&&...) noexcept = 0;
+      constexpr virtual void complete(CPO, Args &&...) noexcept = 0;
 
      protected:
       constexpr ~_virt_completion() = default;
     };
 
-    template <class Sigs, class Env>
+    template <class... Queries>
+    struct _env_of_queries
+    {};
+
+    template <class Return, class Query, class... Args, bool NoThrow>
+    struct _env_of_queries<Return(Query, Args...) noexcept(NoThrow)>
+    {
+      virtual Return query(Query query, Args &&...args) const noexcept(NoThrow) = 0;
+    };
+
+    template <class Return, class Query, class... Args, bool NoThrow, class... Queries>
+    struct _env_of_queries<Return(Query, Args...) noexcept(NoThrow), Queries...>
+      : _env_of_queries<Queries...>
+    {
+      _env_of_queries() = default;
+
+      _env_of_queries(_env_of_queries &&) = delete;
+
+      using _env_of_queries<Queries...>::query;
+
+      virtual Return query(Query query, Args &&...args) const noexcept(NoThrow) = 0;
+
+     protected:
+      ~_env_of_queries() = default;
+    };
+
+    template <class Base, class Derived, class... Queries>
+    struct _delegate_env_base;
+
+    template <class Base, class Derived>
+    struct _delegate_env_base<Base, Derived> : public Base
+    {};
+
+    template <class Base,
+              class Derived,
+              class Return,
+              class Query,
+              class... Args,
+              bool NoThrow,
+              class... Queries>
+    struct _delegate_env_base<Base, Derived, Return(Query, Args...) noexcept(NoThrow), Queries...>
+      : _delegate_env_base<Base, Derived, Queries...>
+    {
+      using query_base = _qry_detail::query<Return(Query, Args...) noexcept>;
+
+      Return query(Query qry, Args &&...args) const noexcept(NoThrow) final
+      {
+        auto &delegate = **static_cast<Derived const *>(this);
+        return __query<Query>()(delegate, std::forward<Args>(args)...);
+      }
+    };
+
+    template <class Queries>
+    struct _delegate_env;
+
+    template <>
+    struct _delegate_env<queries<>>
+      : _delegate_env_base<_env_of_queries<>, _delegate_env<queries<>>>
+    {
+      using delegate_t = _env_of_queries<>;
+
+      explicit _delegate_env(delegate_t const &delegate) noexcept
+        : delegate_(std::addressof(delegate))
+      {}
+
+     private:
+      delegate_t const *delegate_;
+
+      template <class, class, class...>
+      friend class _delegte_env_base;
+
+      delegate_t const &operator*() const noexcept
+      {
+        return *delegate_;
+      }
+    };
+
+    template <class... Queries>
+    struct _delegate_env<queries<Queries...>>
+      : _delegate_env_base<_env_of_queries<Queries...>,
+                           _delegate_env<queries<Queries...>>,
+                           Queries...>
+    {
+      using delegate_t = _env_of_queries<Queries...>;
+
+      explicit _delegate_env(delegate_t const &delegate) noexcept
+        : delegate_(std::addressof(delegate))
+      {}
+
+      //using _delegate_env_base<_env_of_queries<queries
+     private:
+      delegate_t const *delegate_;
+
+      template <class, class, class...>
+      friend class _delegate_env_base;
+
+      delegate_t const &operator*() const noexcept
+      {
+        return *delegate_;
+      }
+    };
+
+    template <class Sigs, class Queries>
     struct _virt_completions;
 
-    template <class... Sigs, class Env>
-    struct _virt_completions<completion_signatures<Sigs...>, Env> : _virt_completion<Sigs>...
+    template <class... Sigs, class... Queries>
+    struct _virt_completions<completion_signatures<Sigs...>, queries<Queries...>>
+      : _virt_completion<Sigs>...
+      , _env_of_queries<Queries...>
     {
       constexpr _virt_completions() = default;
 
-      _virt_completions(_virt_completions&&) = delete;
+      _virt_completions(_virt_completions &&) = delete;
 
       using _virt_completion<Sigs>::complete...;
 
-      virtual Env get_env() const noexcept = 0;
+      constexpr _delegate_env<queries<Queries...>> get_env() const noexcept
+      {
+        return _delegate_env<queries<Queries...>>(*this);
+      }
 
      protected:
       constexpr ~_virt_completions() = default;
     };
 
-    template <class Sigs, class Env>
+    template <class Sigs, class Queries>
     class _func_rcvr;
 
-    template <class... Sigs, class Env>
-    class _func_rcvr<completion_signatures<Sigs...>, Env>
+    template <class... Sigs, class Queries>
+    class _func_rcvr<completion_signatures<Sigs...>, Queries>
     {
-      using completer_t = _virt_completions<completion_signatures<Sigs...>, Env>;
+      using completer_t = _virt_completions<completion_signatures<Sigs...>, Queries>;
 
-      completer_t* completer_;
+      completer_t *completer_;
 
      public:
       using receiver_concept = receiver_tag;
 
-      constexpr explicit _func_rcvr(completer_t& completer) noexcept
+      constexpr explicit _func_rcvr(completer_t &completer) noexcept
         : completer_(std::addressof(completer))
       {}
 
       template <class Error>
-      constexpr void set_error(Error&& err) && noexcept
+      constexpr void set_error(Error &&err) && noexcept
         requires requires { this->completer_->complete(set_error_t{}, std::forward<Error>(err)); }
       {
         this->completer_->complete(set_error_t{}, std::forward<Error>(err));
@@ -144,7 +285,7 @@ namespace experimental::execution
       }
 
       template <class... Values>
-      constexpr void set_value(Values&&... values) && noexcept
+      constexpr void set_value(Values &&...values) && noexcept
         requires requires {
           this->completer_->complete(set_value_t{}, std::forward<Values>(values)...);
         }
@@ -152,7 +293,7 @@ namespace experimental::execution
         this->completer_->complete(set_value_t{}, std::forward<Values>(values)...);
       }
 
-      constexpr auto get_env() const noexcept -> env_of_t<completer_t>
+      constexpr auto get_env() const noexcept -> _delegate_env<Queries>
       {
         return STDEXEC::get_env(*completer_);
       }
@@ -162,7 +303,7 @@ namespace experimental::execution
     {
       constexpr _base_op() = default;
 
-      _base_op(_base_op&&) = delete;
+      _base_op(_base_op &&) = delete;
 
       constexpr virtual ~_base_op() = default;
 
@@ -172,13 +313,13 @@ namespace experimental::execution
     template <class Sender, class Receiver, class Allocator>
     struct _derived_op : _base_op
     {
-      constexpr explicit _derived_op(Sender&& sndr, Receiver rcvr, Allocator const & alloc)
+      constexpr explicit _derived_op(Sender &&sndr, Receiver rcvr, Allocator const &alloc)
         noexcept(std::is_nothrow_invocable_v<connect_t, Sender, Receiver>)
         : op_(connect(std::forward<Sender>(sndr), std::move(rcvr)))
         , alloc_(alloc)
       {}
 
-      _derived_op(_derived_op&&) = delete;
+      _derived_op(_derived_op &&) = delete;
 
       constexpr ~_derived_op() final = default;
 
@@ -187,7 +328,7 @@ namespace experimental::execution
         ::STDEXEC::start(op_);
       }
 
-      static constexpr void operator delete(_derived_op* p, std::destroying_delete_t)
+      static constexpr void operator delete(_derived_op *p, std::destroying_delete_t)
       {
         using traits = std::allocator_traits<Allocator>::template rebind_traits<_derived_op>;
 
@@ -213,7 +354,7 @@ namespace experimental::execution
     struct _func_op_completion<Base, Derived, CPO(Args...), Sigs...>
       : _func_op_completion<Base, Derived, Sigs...>
     {
-      void complete(CPO, Args&&... args) noexcept final
+      void complete(CPO, Args &&...args) noexcept final
       {
         // This seems like it ought to be true, but it fails...
         //
@@ -221,40 +362,60 @@ namespace experimental::execution
         // during constraint satisfaction testing.
         //
         // static_assert(std::derived_from<_func_op_completion, Derived>);
-        auto& rcvr = static_cast<Derived*>(this)->rcvr_;
+        auto &rcvr = static_cast<Derived *>(this)->rcvr_;
         CPO{}(std::move(rcvr), std::forward<Args>(args)...);
       }
     };
 
-    template <class Receiver, class Sigs, class Env>
+    template <class Base, class Derived, class Queries>
+    struct _func_op_queries;
+
+    template <class Base, class Derived>
+    struct _func_op_queries<Base, Derived, queries<>> : Base
+    {};
+
+    template <class Base,
+              class Derived,
+              class Return,
+              class Query,
+              class... Args,
+              bool NoThrow,
+              class... Queries>
+    struct _func_op_queries<Base,
+                            Derived,
+                            queries<Return(Query, Args...) noexcept(NoThrow), Queries...>>
+      : _func_op_queries<Base, Derived, queries<Queries...>>
+    {
+      Return query(Query, Args &&...args) const noexcept(NoThrow) final
+      {
+        using delegate_t = _qry_detail::query<Return(Args...) noexcept(NoThrow)>;
+
+        auto const &rcvr = static_cast<Derived const *>(this)->rcvr_;
+        return __query<Query>()(STDEXEC::get_env(rcvr), std::forward<Args>(args)...);
+      }
+    };
+
+    template <class Receiver, class Sigs, class Queries>
     class _func_op;
 
-    template <class Receiver, class... Sigs, class Env>
-    class _func_op<Receiver, completion_signatures<Sigs...>, Env>
-      : private _func_op_completion<_virt_completions<completion_signatures<Sigs...>, Env>,
-                                    _func_op<Receiver, completion_signatures<Sigs...>, Env>,
-                                    Sigs...>
+    template <class Receiver, class... Sigs, class Queries>
+    class _func_op<Receiver, completion_signatures<Sigs...>, Queries>
+      : _func_op_completion<
+          _func_op_queries<_virt_completions<completion_signatures<Sigs...>, Queries>,
+                           _func_op<Receiver, completion_signatures<Sigs...>, Queries>,
+                           Queries>,
+          _func_op<Receiver, completion_signatures<Sigs...>, Queries>,
+          Sigs...>
     {
-      std::unique_ptr<_base_op> op_;
       [[no_unique_address]]
-      Receiver rcvr_;
+      Receiver                  rcvr_;
+      std::unique_ptr<_base_op> op_;
 
       template <class B, class D, class... S>
       friend struct _func_op_completion;
 
-      constexpr Env get_env() const noexcept final
-      {
-        using RcvrEnv = env_of_t<Receiver>;
-
-        if constexpr (std::constructible_from<Env, RcvrEnv>)
-        {
-          return Env(::STDEXEC::get_env(rcvr_));
-        }
-        else
-        {
-          return {};
-        }
-      }
+      template <class, class, class>
+      friend struct _func_op_queries;
 
      public:
       using operation_state_concept = operation_state_tag;
@@ -262,9 +423,10 @@ namespace experimental::execution
       template <class Factory>
       constexpr _func_op(Receiver rcvr, Factory factory)
         : rcvr_(std::move(rcvr))
-        , op_(factory(_func_rcvr<completion_signatures<Sigs...>, Env>(*this))){};
+        , op_(factory(_func_rcvr<completion_signatures<Sigs...>, Queries>(*this)))
+      {}
 
-      _func_op(_func_op&&) = delete;
+      _func_op(_func_op &&) = delete;
 
       constexpr ~_func_op() = default;
 
@@ -275,7 +437,7 @@ namespace experimental::execution
     };
 
     template <class Env>
-    constexpr auto choose_frame_allocator(Env const & env) noexcept
+    constexpr auto choose_frame_allocator(Env const &env) noexcept
     {
       if constexpr (requires { get_frame_allocator(env); })
       {
@@ -291,14 +453,14 @@ namespace experimental::execution
       }
     }
 
-    template <class Args, class Sigs, class Env>
+    template <class Args, class Sigs, class Queries>
     class _func_impl;
 
-    template <class SndrCncpt, class... Args, class... Sigs, class Env>
-    class _func_impl<SndrCncpt(Args...), completion_signatures<Sigs...>, Env>
+    template <class SndrCncpt, class... Args, class... Sigs, class... Queries>
+    class _func_impl<SndrCncpt(Args...), completion_signatures<Sigs...>, queries<Queries...>>
     {
-      std::unique_ptr<_base_op> (*factory_)(_func_rcvr<completion_signatures<Sigs...>, Env>,
-                                            Args&&...);
+      std::unique_ptr<_base_op> (
+        *factory_)(_func_rcvr<completion_signatures<Sigs...>, queries<Queries...>>, Args &&...);
       [[no_unique_address]]
       std::tuple<Args...> args_;
 
@@ -310,15 +472,15 @@ namespace experimental::execution
               && std::constructible_from<Factory>               //
               && STDEXEC::__callable<Factory, Args...>
               && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
-                                    _func_rcvr<completion_signatures<Sigs...>, Env>>
-      constexpr explicit(sizeof...(Args) == 0) _func_impl(Args&&... args, Factory&& factory)
+                                    _func_rcvr<completion_signatures<Sigs...>, queries<Queries...>>>
+      constexpr explicit(sizeof...(Args) == 0) _func_impl(Args &&...args, Factory &&factory)
         noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
         : args_(std::forward<Args>(args)...)
       {
         using sender_t   = std::invoke_result_t<Factory, Args...>;
-        using receiver_t = _func_rcvr<completion_signatures<Sigs...>, Env>;
+        using receiver_t = _func_rcvr<completion_signatures<Sigs...>, queries<Queries...>>;
 
-        factory_ = [](receiver_t rcvr, Args&&... args) -> std::unique_ptr<_base_op>
+        factory_ = [](receiver_t rcvr, Args &&...args) -> std::unique_ptr<_base_op>
         {
           // the type of the allocator provided by the receiver's environment
           using alloc_t = decltype(choose_frame_allocator(get_env(rcvr)));
@@ -333,10 +495,10 @@ namespace experimental::execution
           // finally, the allocator traits for an allocator that can allocate an op_t
           using traits = traits_t::template rebind_traits<op_t>;
 
-	  // ...and the allocator itself
+          // ...and the allocator itself
           typename traits::allocator_type alloc(choose_frame_allocator(get_env(rcvr)));
 
-          auto* op = traits::allocate(alloc, 1);
+          auto *op = traits::allocate(alloc, 1);
 
           __scope_guard guard{[&]() noexcept { traits::deallocate(alloc, op, 1); }};
 
@@ -354,21 +516,34 @@ namespace experimental::execution
         };
       }
 
-      template <class Sender, class /* Env */>
-      static consteval completion_signatures<Sigs...> get_completion_signatures() noexcept
+      template <class Sender, class RcvrEnv>
+      static consteval auto get_completion_signatures() noexcept
       {
-        // TODO: validate that the Env passed here is compatible with the class-level Env
-        return {};
+        static_assert(STDEXEC_IS_BASE_OF(_func_impl, __decay_t<Sender>));
+        //static_assert(std::constructible_from<Env, RcvrEnv const &>);
+
+        //Env env{RcvrEnv{}};
+
+        //if constexpr (std::constructible_from<Env, RcvrEnv const &>)
+        {
+          return completion_signatures<Sigs...>{};
+        }
+        //else
+        //{
+        // TODO: make this error accurate
+        //return __throw_compile_time_error(__unrecognized_sender_error_t<Sender, RcvrEnv>());
+        //}
       }
 
       template <class Receiver>
-      constexpr _func_op<Receiver, completion_signatures<Sigs...>, Env> connect(Receiver rcvr)
+      constexpr _func_op<Receiver, completion_signatures<Sigs...>, queries<Queries...>>
+      connect(Receiver rcvr)
       {
         return {std::move(rcvr),
                 [this](auto rcvr)
                 {
                   return std::apply(
-                    [&rcvr, this](Args&&... args)
+                    [&rcvr, this](Args &&...args)
                     { return factory_(std::move(rcvr), std::forward<Args>(args)...); },
                     std::move(args_));
                 }};
@@ -426,11 +601,11 @@ namespace experimental::execution
   struct function<Return(Args...) noexcept(NoThrow)>
     : _func::_func_impl<STDEXEC::sender_tag(Args...),
                         _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
-                        STDEXEC::env<>>
+                        queries<>>
   {
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
                                    _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
-                                   STDEXEC::env<>>;
+                                   queries<>>;
 
     using base::base;
   };
@@ -438,34 +613,37 @@ namespace experimental::execution
   template <class... Args, class Sigs>
     requires STDEXEC::__is_instance_of<Sigs, STDEXEC::completion_signatures>
   struct function<STDEXEC::sender_tag(Args...), Sigs>
-    : _func::_func_impl<STDEXEC::sender_tag(Args...), Sigs, STDEXEC::env<>>
+    : _func::_func_impl<STDEXEC::sender_tag(Args...), Sigs, queries<>>
   {
-    using base = _func::_func_impl<STDEXEC::sender_tag(Args...), Sigs, STDEXEC::env<>>;
+    using base = _func::_func_impl<STDEXEC::sender_tag(Args...), Sigs, queries<>>;
 
     using base::base;
   };
 
-  template <class Return, class... Args, bool NoThrow, class Env>
-    requires STDEXEC::__is_not_instance_of<Env, STDEXEC::completion_signatures>
-  struct function<Return(Args...) noexcept(NoThrow), Env>
+  template <class Return, class... Args, bool NoThrow, class... Queries>
+  struct function<Return(Args...) noexcept(NoThrow), queries<Queries...>>
     : _func::_func_impl<STDEXEC::sender_tag(Args...),
                         _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
-                        Env>
+                        queries<Queries...>>
   {
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
                                    _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
-                                   Env>;
+                                   queries<Queries...>>;
 
     using base::base;
   };
 
-  template <class... Args, class... Sigs, class Env>
-    requires STDEXEC::__is_not_instance_of<Env, STDEXEC::completion_signatures>
-  struct function<STDEXEC::sender_tag(Args...), STDEXEC::completion_signatures<Sigs...>, Env>
-    : _func::_func_impl<STDEXEC::sender_tag(Args...), STDEXEC::completion_signatures<Sigs...>, Env>
+  template <class... Args, class... Sigs, class... Queries>
+  struct function<STDEXEC::sender_tag(Args...),
+                  STDEXEC::completion_signatures<Sigs...>,
+                  queries<Queries...>>
+    : _func::_func_impl<STDEXEC::sender_tag(Args...),
+                        STDEXEC::completion_signatures<Sigs...>,
+                        queries<Queries...>>
   {
-    using base =
-      _func::_func_impl<STDEXEC::sender_tag(Args...), STDEXEC::completion_signatures<Sigs...>, Env>;
+    using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
+                                   STDEXEC::completion_signatures<Sigs...>,
+                                   queries<Queries...>>;
 
     using base::base;
   };
diff --git a/test/exec/test_function.cpp b/test/exec/test_function.cpp
index f927426b5..3fedbd7fe 100644
--- a/test/exec/test_function.cpp
+++ b/test/exec/test_function.cpp
@@ -46,9 +46,11 @@ namespace
     exec::function<ex::sender_tag(), ex::completion_signatures<ex::set_stopped_t()>> onlystopped(
       []() noexcept { return ex::just_stopped(); });
 
-    exec::function<void(), ex::env<>> trivialCustomEnv([]() noexcept { return ex::just(); });
+    exec::function<void(), exec::queries<>> trivialCustomEnv([]() noexcept { return ex::just(); });
 
-    exec::function<ex::sender_tag(int), ex::completion_signatures<ex::set_value_t()>, ex::env<>>
+    exec::function<ex::sender_tag(int),
+                   ex::completion_signatures<ex::set_value_t()>,
+                   exec::queries<>>
       totalControl(5, [](int) noexcept { return ex::just(); });
 
     STATIC_REQUIRE(STDEXEC::sender<decltype(voidSndr)>);
@@ -105,10 +107,12 @@ namespace
   TEST_CASE("exec::function forwards get_frame_allocator", "[types][function]")
   {
     // TODO: you probably shouldn't have to specify the frame allocator query like this
-    using Env =
-      ex::env<ex::prop<exec::get_frame_allocator_t, std::pmr::polymorphic_allocator<std::byte>>>;
+    //using Env =
+      //ex::env<ex::prop<exec::get_frame_allocator_t, std::pmr::polymorphic_allocator<std::byte>>>;
+    using Queries = exec::queries<std::pmr::polymorphic_allocator<std::byte>(
+      exec::get_frame_allocator_t) noexcept>;
 
-    exec::function<bool() noexcept, Env> sndr(
+    exec::function<bool() noexcept, Queries> sndr(
       []() noexcept
       {
         return ex::read_env(exec::get_frame_allocator)

From 1094cc5151afe5e6620669ce6e2046a2bb3b63ac Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Tue, 21 Apr 2026 16:40:22 -0700
Subject: [PATCH 15/20] Tidy up and add comments

This diff does some tidying and adds documentation. There are still some
TODOs, but this is in good enough shape that I can start sharing it, I
think.
---
 include/exec/function.hpp | 335 +++++++++++++++++++++-----------------
 1 file changed, 188 insertions(+), 147 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 28fda5db2..757e7651c 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -49,6 +49,8 @@
 // the frame allocator from the environment without relying on TLS.
 namespace experimental::execution
 {
+  // A forwarding query for a "frame allocator", to be used for dynamically allocating
+  // the operation states of senders type-erased by exec::function.
   struct get_frame_allocator_t : STDEXEC::__query<get_frame_allocator_t>
   {
     using STDEXEC::__query<get_frame_allocator_t>::operator();
@@ -76,72 +78,51 @@ namespace experimental::execution
 
   namespace _qry_detail
   {
-    using namespace STDEXEC;
-
-    template <bool NoThrow, class Env, class Query, class... Args>
-    concept _conditionally_nothrow_queryable_with =
-      (!NoThrow && __queryable_with<Env const &, Query, Args...>)
-      || __nothrow_queryable_with<Env const &, Query, Args...>;
-
-    template <class Expected, bool NoThrow, class Env, class Query, class... Args>
-    concept _query_result_convertible_to =
-      (!NoThrow && std::is_convertible_v<__query_result_t<Env const &, Query, Args...>, Expected>)
-      || std::is_nothrow_convertible_v<__query_result_t<Env const &, Query, Args...>, Expected>;
-
     template <class Sig>
-    struct query;
+    inline constexpr bool is_query_function_v = false;
 
     template <class Return, class Query, class... Args, bool NoThrow>
-    struct query<Return(Query, Args...) noexcept(NoThrow)>
-    {
-     protected:
-      template <class Env>
-        requires _conditionally_nothrow_queryable_with<NoThrow, Env, Query, Args...>
-              && _query_result_convertible_to<Return, NoThrow, Env, Query, Args...>
-      static Return query_delegate(Env const &env, Query query, Args &&...args) noexcept(NoThrow)
-      {
-        return __query<Query>()(env, std::forward<Args>(args)...);
-      }
-    };
+    inline constexpr bool is_query_function_v<Return(Query, Args...) noexcept(NoThrow)> = true;
   }  // namespace _qry_detail
 
+  // a "type list" for bundling together function type representing queries to support in
+  // a type-erased environment. All of the types in Queries... must be (possibly noexcept)
+  // function types. For example:
+  //
+  //   queries<
+  //     std::execution::inline_stop_token(std::execution::get_stop_token_t) noexcept,
+  //     std::pmr::polymorphic_allocator<std::byte>(std::execution::get_allocator_t)
+  //   >
   template <class... Queries>
-  struct queries : _qry_detail::query<Queries>...
+    requires(_qry_detail::is_query_function_v<Queries> && ...)
+  struct queries
   {};
 
   namespace _func
   {
     using namespace STDEXEC;
 
-    template <class Sig>
-    struct _virt_completion;
-
-    template <class CPO, class... Args>
-    struct _virt_completion<CPO(Args...)>
-    {
-      constexpr _virt_completion() = default;
-
-      _virt_completion(_virt_completion &&) = delete;
-
-      constexpr virtual void complete(CPO, Args &&...) noexcept = 0;
-
-     protected:
-      constexpr ~_virt_completion() = default;
-    };
-
+    // a recursively-defined type with a vtable containing one virtual function for
+    // each query in Queries...
+    //
+    // the base template is an empty class, representing the empty set of queries.
     template <class... Queries>
     struct _env_of_queries
     {};
 
+    // a special case in the recursion: when there is only one query in the pack, there's
+    // no base implementation of query to put in the using statement
     template <class Return, class Query, class... Args, bool NoThrow>
     struct _env_of_queries<Return(Query, Args...) noexcept(NoThrow)>
     {
       virtual Return query(Query query, Args &&...args) const noexcept(NoThrow) = 0;
     };
 
+    // the recursive case that declares the named query as a pure virtual member function
+    // and inherits the rest of the required queries through inheritance
     template <class Return, class Query, class... Args, bool NoThrow, class... Queries>
     struct _env_of_queries<Return(Query, Args...) noexcept(NoThrow), Queries...>
-      : _env_of_queries<Queries...>
+      : private _env_of_queries<Queries...>
     {
       _env_of_queries() = default;
 
@@ -155,87 +136,64 @@ namespace experimental::execution
       ~_env_of_queries() = default;
     };
 
-    template <class Base, class Derived, class... Queries>
-    struct _delegate_env_base;
-
-    template <class Base, class Derived>
-    struct _delegate_env_base<Base, Derived> : public Base
-    {};
-
-    template <class Base,
-              class Derived,
-              class Return,
-              class Query,
-              class... Args,
-              bool NoThrow,
-              class... Queries>
-    struct _delegate_env_base<Base, Derived, Return(Query, Args...) noexcept(NoThrow), Queries...>
-      : _delegate_env_base<Base, Derived, Queries...>
-    {
-      using query_base = _qry_detail::query<Return(Query, Args...) noexcept>;
-
-      Return query(Query qry, Args &&...args) const noexcept(NoThrow) final
-      {
-        auto &delegate = **static_cast<Derived const *>(this);
-        return __query<Query>()(delegate, std::forward<Args>(args)...);
-      }
-    };
-
-    template <class Queries>
-    struct _delegate_env;
-
-    template <>
-    struct _delegate_env<queries<>>
-      : _delegate_env_base<_env_of_queries<>, _delegate_env<queries<>>>
+    // an environment type that delegates query to an _env_of_queries<Queries...> so that the
+    // environment type that we traffic in is cheaply copyable
+    template <class... Queries>
+    struct _delegate_env
     {
-      using delegate_t = _env_of_queries<>;
+      using delegate_t = _env_of_queries<Queries...>;
 
       explicit _delegate_env(delegate_t const &delegate) noexcept
         : delegate_(std::addressof(delegate))
       {}
 
+      template <class Query, class... Args>
+        requires __queryable_with<delegate_t, Query, Args...>
+      constexpr auto query(Query, Args &&...args) const
+        noexcept(__nothrow_queryable_with<delegate_t, Query, Args...>)
+          -> __query_result_t<delegate_t, Query, Args...>
+      {
+        return __query<Query>()(*delegate_, std::forward<Args>(args)...);
+      }
+
      private:
       delegate_t const *delegate_;
+    };
 
-      template <class, class, class...>
-      friend class _delegte_env_base;
+    // in the base case, there's no need to store a pointer
+    template <>
+    struct _delegate_env<>
+    {
+      using delegate_t = _env_of_queries<>;
 
-      delegate_t const &operator*() const noexcept
-      {
-        return *delegate_;
-      }
+      explicit _delegate_env(delegate_t const &) noexcept {}
     };
 
-    template <class... Queries>
-    struct _delegate_env<queries<Queries...>>
-      : _delegate_env_base<_env_of_queries<Queries...>,
-                           _delegate_env<queries<Queries...>>,
-                           Queries...>
-    {
-      using delegate_t = _env_of_queries<Queries...>;
+    template <class Sig>
+    struct _virt_completion;
 
-      explicit _delegate_env(delegate_t const &delegate) noexcept
-        : delegate_(std::addressof(delegate))
-      {}
+    // a vtable entry representing a receiver completion function; CPO should be a completion
+    // function (e.g. set_Value_t), and Args... is the expected argument list.
+    template <class CPO, class... Args>
+    struct _virt_completion<CPO(Args...)>
+    {
+      constexpr _virt_completion() = default;
 
-      //using _delegate_env_base<_env_of_queries<queries
-     private:
-      delegate_t const *delegate_;
+      _virt_completion(_virt_completion &&) = delete;
 
-      template <class, class, class...>
-      friend class _delegate_env_base;
+      constexpr virtual void complete(CPO, Args &&...) noexcept = 0;
 
-      delegate_t const &operator*() const noexcept
-      {
-        return *delegate_;
-      }
+     protected:
+      constexpr ~_virt_completion() = default;
     };
 
-    template <class Sigs, class Queries>
+    template <class Sigs, class... Queries>
     struct _virt_completions;
 
+    // a class template that bundles together a pure virtual completion function for each
+    // of the specified completion functions, and provides an implementation of get_env
     template <class... Sigs, class... Queries>
-    struct _virt_completions<completion_signatures<Sigs...>, queries<Queries...>>
+    struct _virt_completions<completion_signatures<Sigs...>, Queries...>
       : _virt_completion<Sigs>...
       , _env_of_queries<Queries...>
     {
@@ -243,24 +201,33 @@ namespace experimental::execution
 
       _virt_completions(_virt_completions &&) = delete;
 
+      // this will complain if sizeof...(Sigs) == 0, but a sender with no completions
+      // isn't super useful...
       using _virt_completion<Sigs>::complete...;
 
-      constexpr _delegate_env<queries<Queries...>> get_env() const noexcept
+      constexpr _delegate_env<Queries...> get_env() const noexcept
       {
-        return _delegate_env<queries<Queries...>>(*this);
+        return _delegate_env<Queries...>(*this);
       }
 
      protected:
       constexpr ~_virt_completions() = default;
     };
 
-    template <class Sigs, class Queries>
+    template <class Sigs, class... Queries>
     class _func_rcvr;
 
-    template <class... Sigs, class Queries>
-    class _func_rcvr<completion_signatures<Sigs...>, Queries>
+    // a type-erased receiver expecting to be completed by one of the completions specified
+    // in Sigs..., and providing an environment that supports the queries specified in
+    // Queries...
+    //
+    // this is the receiver type that is passed into the sender being type-erased by a
+    // function<...>, and it forwards completions to the concrete receiver through the
+    // internal completer_ pointer
+    template <class... Sigs, class... Queries>
+    class _func_rcvr<completion_signatures<Sigs...>, Queries...>
     {
-      using completer_t = _virt_completions<completion_signatures<Sigs...>, Queries>;
+      using completer_t = _virt_completions<completion_signatures<Sigs...>, Queries...>;
 
       completer_t *completer_;
 
@@ -273,32 +240,31 @@ namespace experimental::execution
 
       template <class Error>
       constexpr void set_error(Error &&err) && noexcept
-        requires requires { this->completer_->complete(set_error_t{}, std::forward<Error>(err)); }
+        requires requires { completer_->complete(set_error_t{}, std::forward<Error>(err)); }
       {
-        this->completer_->complete(set_error_t{}, std::forward<Error>(err));
+        completer_->complete(set_error_t{}, std::forward<Error>(err));
       }
 
       constexpr void set_stopped() && noexcept
-        requires requires { this->completer_->complete(set_stopped_t{}); }
+        requires requires { completer_->complete(set_stopped_t{}); }
       {
-        this->completer_->complete(set_stopped_t{});
+        completer_->complete(set_stopped_t{});
       }
 
       template <class... Values>
       constexpr void set_value(Values &&...values) && noexcept
-        requires requires {
-          this->completer_->complete(set_value_t{}, std::forward<Values>(values)...);
-        }
+        requires requires { completer_->complete(set_value_t{}, std::forward<Values>(values)...); }
       {
-        this->completer_->complete(set_value_t{}, std::forward<Values>(values)...);
+        completer_->complete(set_value_t{}, std::forward<Values>(values)...);
       }
 
-      constexpr auto get_env() const noexcept -> _delegate_env<Queries>
+      constexpr auto get_env() const noexcept -> _delegate_env<Queries...>
       {
         return STDEXEC::get_env(*completer_);
       }
     };
 
+    // the type-erased operation state type that supports starting and destruction
     struct _base_op
     {
       constexpr _base_op() = default;
@@ -310,6 +276,9 @@ namespace experimental::execution
       constexpr virtual void start() & noexcept = 0;
     };
 
+    // the operation state resulting from connecting a sender being erased by a function<...>
+    // with a _func_rcvr<...>; inherits from _base_op, and provides a class-specific override
+    // of operator delete that invokes the allocator deallocation protocol
     template <class Sender, class Receiver, class Allocator>
     struct _derived_op : _base_op
     {
@@ -328,6 +297,10 @@ namespace experimental::execution
         ::STDEXEC::start(op_);
       }
 
+      // objects of this type are allocated with an allocator of type Allocator so they need
+      // to be deallocated using the same allocator; providing a class-specific overload of
+      // a destroying operator delete allows us to store the relevant allocator inside the
+      // to-be-destroyed object and retrieve it before running the destructor
       static constexpr void operator delete(_derived_op *p, std::destroying_delete_t)
       {
         using traits = std::allocator_traits<Allocator>::template rebind_traits<_derived_op>;
@@ -343,13 +316,20 @@ namespace experimental::execution
       Allocator alloc_;
     };
 
+    // a recursive implementation of Base, which is expected to inherit from
+    // _virt_completions
     template <class Base, class Derived, class... Sigs>
     struct _func_op_completion;
 
+    // the base case of the recursive implementation; all subclasses of this type have,
+    // together, overridden all the virtual functions in Base so now we just need to
+    // inherit from Base to ensure those virtual functions exist to be overridden
     template <class Base, class Derived>
     struct _func_op_completion<Base, Derived> : Base
     {};
 
+    // the recursive case, which implements a single overload of complete and delegates
+    // the implementation of all remaining overloads to the base class
     template <class Base, class Derived, class CPO, class... Args, class... Sigs>
     struct _func_op_completion<Base, Derived, CPO(Args...), Sigs...>
       : _func_op_completion<Base, Derived, Sigs...>
@@ -358,22 +338,31 @@ namespace experimental::execution
       {
         // This seems like it ought to be true, but it fails...
         //
-        // Some testing shows it's being evaluated when Derive is incomplete
+        // Some testing shows it's being evaluated when Derived is incomplete
         // during constraint satisfaction testing.
         //
         // static_assert(std::derived_from<_func_op_completion, Derived>);
+        //
+        // Consider: what if _func_op_completion<Base, Derived> (i.e. the base case of
+        //           this recursive class hierarchy) owned the receiver? We could avoid
+        //           CRTP and just use this->rcvr_, maybe.
         auto &rcvr = static_cast<Derived *>(this)->rcvr_;
         CPO{}(std::move(rcvr), std::forward<Args>(args)...);
       }
     };
 
-    template <class Base, class Derived, class Queries>
+    // a recursive implementation of all the queries in Queries...
+    template <class Base, class Derived, class... Queries>
     struct _func_op_queries;
 
+    // the base case of the recursive implementation; there are no more queries to
+    // implement so just inherit from Base
     template <class Base, class Derived>
-    struct _func_op_queries<Base, Derived, queries<>> : Base
+    struct _func_op_queries<Base, Derived> : Base
     {};
 
+    // the recursive case, which implements a single query overload and delegates the
+    // implementation of the remaining overloads to the base class
     template <class Base,
               class Derived,
               class Return,
@@ -381,40 +370,49 @@ namespace experimental::execution
               class... Args,
               bool NoThrow,
               class... Queries>
-    struct _func_op_queries<Base,
-                            Derived,
-                            queries<Return(Query, Args...) noexcept(NoThrow), Queries...>>
-      : _func_op_queries<Base, Derived, queries<Queries...>>
+    struct _func_op_queries<Base, Derived, Return(Query, Args...) noexcept(NoThrow), Queries...>
+      : _func_op_queries<Base, Derived, Queries...>
     {
       Return query(Query, Args &&...args) const noexcept(NoThrow) final
       {
-        using delegate_t = _qry_detail::query<Return(Args...) noexcept(NoThrow)>;
-
+        // the idea of storing the receiver in the base class could help here, too, but
+        // we'd need to be careful about which class template is actually the base class
         auto const &rcvr = static_cast<Derived const *>(this)->rcvr_;
         return __query<Query>()(STDEXEC::get_env(rcvr), std::forward<Args>(args)...);
       }
     };
 
-    template <class Receiver, class Sigs, class Queries>
+    template <class Receiver, class Sigs, class... Queries>
     class _func_op;
 
-    template <class Receiver, class... Sigs, class Queries>
-    class _func_op<Receiver, completion_signatures<Sigs...>, Queries>
+    // the concrete operation state resulting from connecting a function<...> to a concrete
+    // receiver of type Receiver. this type manages a dynamically-allocated _derived_op instance,
+    // which is the type-erased operation state resulting from connecting the type-erased sender
+    // to a _func_rcvr
+    template <class Receiver, class... Sigs, class... Queries>
+    class _func_op<Receiver, completion_signatures<Sigs...>, Queries...>
       : _func_op_completion<
-          _func_op_queries<_virt_completions<completion_signatures<Sigs...>, Queries>,
-                           _func_op<Receiver, completion_signatures<Sigs...>, Queries>,
-                           Queries>,
-          _func_op<Receiver, completion_signatures<Sigs...>, Queries>,
+          _func_op_queries<_virt_completions<completion_signatures<Sigs...>, Queries...>,
+                           _func_op<Receiver, completion_signatures<Sigs...>, Queries...>,
+                           Queries...>,
+          _func_op<Receiver, completion_signatures<Sigs...>, Queries...>,
           Sigs...>
     {
+      // rcvr_ has to be initialized before op_ because our implementation of get_env
+      // is empirically accessed during our constructor and depends on rcvr_ being initialized
       [[no_unique_address]]
-      Receiver                  rcvr_;
+      Receiver rcvr_;
+      // the default deleter is OK because we've virtualized operator delete to invoke
+      // the allocator-based deallocation logic that's necessary to properly support
+      // a user-provided frame allocator
       std::unique_ptr<_base_op> op_;
 
-      template <class B, class D, class... S>
+      // these friend declaratiosn allow our CRTP base classes to access rcvr_; they could
+      // disappear if we moved ownership of rcvr_ into the base class object
+      template <class, class, class...>
       friend struct _func_op_completion;
 
-      template <class, class, class>
+      template <class, class, class...>
       friend struct _func_op_queries;
 
      public:
@@ -423,7 +421,7 @@ namespace experimental::execution
       template <class Factory>
       constexpr _func_op(Receiver rcvr, Factory factory)
         : rcvr_(std::move(rcvr))
-        , op_(factory(_func_rcvr<completion_signatures<Sigs...>, Queries>(*this)))
+        , op_(factory(_func_rcvr<completion_signatures<Sigs...>, Queries...>(*this)))
       {}
 
       _func_op(_func_op &&) = delete;
@@ -436,6 +434,9 @@ namespace experimental::execution
       }
     };
 
+    // given the concrete receiver's environment, choose the frame allocator; first choice
+    // is the result of get_frame_allocator(env), second choice is get_allocator(env), and
+    // the default is std::allocator
     template <class Env>
     constexpr auto choose_frame_allocator(Env const &env) noexcept
     {
@@ -456,29 +457,43 @@ namespace experimental::execution
     template <class Args, class Sigs, class Queries>
     class _func_impl;
 
+    // the main implementation of the type-erasing sender function<...>
+    //
+    // SndrCncpt should be std::execution::sender_concept
+    // Args... is the argument types used to construct the erased sender
+    // Sigs... is the supported completion signatures
+    // Queries... is the list of environment queries that must be supported by the eventual
+    //            receiver; it's a pack of function type like Return(Query, Args...) or
+    //            Return(Query, Args...) noexcept. The named query, when given the specified
+    //            arguments, must return a value convertible to Return, and it must be noexcept,
+    //            or not, as appropriate
     template <class SndrCncpt, class... Args, class... Sigs, class... Queries>
     class _func_impl<SndrCncpt(Args...), completion_signatures<Sigs...>, queries<Queries...>>
     {
-      std::unique_ptr<_base_op> (
-        *factory_)(_func_rcvr<completion_signatures<Sigs...>, queries<Queries...>>, Args &&...);
+      // the type-erased sender factory that, when called, constructs the erased sender from
+      // args_ and connects the resulting sender to the provided receiver
+      std::unique_ptr<_base_op> (*factory_)(_func_rcvr<completion_signatures<Sigs...>, Queries...>,
+                                            Args &&...);
       [[no_unique_address]]
       std::tuple<Args...> args_;
 
      public:
       using sender_concept = SndrCncpt;
 
+      // TODO: I only know this works for empty lambdas; figure out whether function pointers
+      //       and/or pointer-to-member functions can be made to work
       template <STDEXEC::__callable<Args...> Factory>
         requires STDEXEC::__not_decays_to<Factory, _func_impl>  //
               && std::constructible_from<Factory>               //
               && STDEXEC::__callable<Factory, Args...>
               && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
-                                    _func_rcvr<completion_signatures<Sigs...>, queries<Queries...>>>
+                                    _func_rcvr<completion_signatures<Sigs...>, Queries...>>
       constexpr explicit(sizeof...(Args) == 0) _func_impl(Args &&...args, Factory &&factory)
         noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
         : args_(std::forward<Args>(args)...)
       {
         using sender_t   = std::invoke_result_t<Factory, Args...>;
-        using receiver_t = _func_rcvr<completion_signatures<Sigs...>, queries<Queries...>>;
+        using receiver_t = _func_rcvr<completion_signatures<Sigs...>, Queries...>;
 
         factory_ = [](receiver_t rcvr, Args &&...args) -> std::unique_ptr<_base_op>
         {
@@ -502,6 +517,9 @@ namespace experimental::execution
 
           __scope_guard guard{[&]() noexcept { traits::deallocate(alloc, op, 1); }};
 
+          // TODO: as mentioned above, Factory must be a stateless lambda, which makes it
+          //       default-constructible like this; this obviously doesn't work if Factory
+          //       is a pointer type
           Factory factory;
 
           traits::construct(alloc,
@@ -516,14 +534,13 @@ namespace experimental::execution
         };
       }
 
-      template <class Sender, class RcvrEnv>
+      template <class Sender, class Env>
       static consteval auto get_completion_signatures() noexcept
       {
         static_assert(STDEXEC_IS_BASE_OF(_func_impl, __decay_t<Sender>));
-        //static_assert(std::constructible_from<Env, RcvrEnv const &>);
-
-        //Env env{RcvrEnv{}};
 
+        // TODO: validate that Env supports all the required queries
+        //
         //if constexpr (std::constructible_from<Env, RcvrEnv const &>)
         {
           return completion_signatures<Sigs...>{};
@@ -535,8 +552,9 @@ namespace experimental::execution
         //}
       }
 
+      // TODO: this assumes rvalue connection; lvalue connection requires thought and tests
       template <class Receiver>
-      constexpr _func_op<Receiver, completion_signatures<Sigs...>, queries<Queries...>>
+      constexpr _func_op<Receiver, completion_signatures<Sigs...>, Queries...>
       connect(Receiver rcvr)
       {
         return {std::move(rcvr),
@@ -550,6 +568,10 @@ namespace experimental::execution
       }
     };
 
+    // given a possibly-noexcept function type like Return(Args...), compute the appropriate
+    // completion_signatures. The result is a set_value overload taking either Return&& or
+    // no args when Return is void, set_stopped, and, when the function type is not noexcept,
+    // set_error(std::exception_ptr)
     template <class Sig>
     struct _sigs_from;
 
@@ -586,7 +608,26 @@ namespace experimental::execution
     using _sigs_from_t = _sigs_from<Sig>::type;
   }  // namespace _func
 
-  // TODO: think about environment forwarding
+  // the user-facing interface to exec::function that supports several different declaration
+  // styles, including:
+  //  - function<int(bar, baz)>: a fallible function from (bar, baz) to int
+  //  - function<int(bar, baz) noexcept>: an infallible function from (bar, baz) to int
+  //  - function<sender_tag(bar, baz), completion_signatures<...>>: a function from (bar, baz)
+  //    that completes in the ways specified by the given specialization of completion_signatures
+  //  - function<int(bar, baz), queries<Return(Query, Args...), ...>: a function from (bar, baz)
+  //    to int that requires the final receiver to have an environment that supports the
+  //    Query query, taking arguments Args..., and returning an object convertible to Return; queries
+  //    may be required to be no-throw by delcaring the function type noexcept
+  //  - function<
+  //        sender_tag(bar, baz),
+  //        completion_signatures<...>,
+  //        queries<Return(Query, Args...)>>: a fully-specified async function that maps (bar, baz)
+  //    to the specified completions, requiring the specified queries in the ultimate receiver's
+  //    environment
+  //
+  // Future: support C-style ellipsis arguments in the function signature to permit type-erased
+  //         arguments as well, like function<int(bar, baz, ...)> (a fallible function from
+  //         (bar, baz) plus unspecified, erased additional arguments to int)
   template <class...>
   struct function;
 

From dc225b81bd1e6df94442145439142b53eb845136 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Tue, 28 Apr 2026 10:25:36 -0700
Subject: [PATCH 16/20] Remove [[no_unique_address]]

Per code review feedback, replace `[[no_unique_address]]` with
`STDEXEC_ATTRIBUTE(no_unique_address)`.
---
 include/exec/function.hpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 757e7651c..d18cc46c0 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -312,7 +312,7 @@ namespace experimental::execution
 
      private:
       connect_result_t<Sender, Receiver> op_;
-      [[no_unique_address]]
+      STDEXEC_ATTRIBUTE(no_unique_address)
       Allocator alloc_;
     };
 
@@ -400,7 +400,7 @@ namespace experimental::execution
     {
       // rcvr_ has to be initialized before op_ because our implementation of get_env
       // is empirically accessed during our constructor and depends on rcvr_ being initialized
-      [[no_unique_address]]
+      STDEXEC_ATTRIBUTE(no_unique_address)
       Receiver rcvr_;
       // the default deleter is OK because we've virtualized operator delete to invoke
       // the allocator-based deallocation logic that's necessary to properly support
@@ -474,7 +474,7 @@ namespace experimental::execution
       // args_ and connects the resulting sender to the provided receiver
       std::unique_ptr<_base_op> (*factory_)(_func_rcvr<completion_signatures<Sigs...>, Queries...>,
                                             Args &&...);
-      [[no_unique_address]]
+      STDEXEC_ATTRIBUTE(no_unique_address)
       std::tuple<Args...> args_;
 
      public:

From a14b6bc432fbbf48c665930cca76e835102e58cb Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Tue, 28 Apr 2026 10:28:22 -0700
Subject: [PATCH 17/20] Clean up the _func_impl constructor

Take @ericniebler's code review feedback to clean up the declaration of
`exec::_func::_func_impl`'s constructor.
---
 include/exec/function.hpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index d18cc46c0..e9eb06f89 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -484,12 +484,12 @@ namespace experimental::execution
       //       and/or pointer-to-member functions can be made to work
       template <STDEXEC::__callable<Args...> Factory>
         requires STDEXEC::__not_decays_to<Factory, _func_impl>  //
-              && std::constructible_from<Factory>               //
+              && STDEXEC::__std::constructible_from<Factory>    //
               && STDEXEC::__callable<Factory, Args...>
               && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
                                     _func_rcvr<completion_signatures<Sigs...>, Queries...>>
-      constexpr explicit(sizeof...(Args) == 0) _func_impl(Args &&...args, Factory &&factory)
-        noexcept((std::is_nothrow_constructible_v<Args, Args> && ...))
+      constexpr explicit _func_impl(Args &&...args, Factory &&factory)
+        noexcept(STDEXEC::__nothrow_move_constructible<Args...>)
         : args_(std::forward<Args>(args)...)
       {
         using sender_t   = std::invoke_result_t<Factory, Args...>;

From eb953d2f3394ee039117494e57b36644c4a65da9 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Tue, 28 Apr 2026 20:32:12 -0700
Subject: [PATCH 18/20] Replace most of exec::function with _any_receiver_ref

This commit replaces the vtable-building shenanigans in `exec::function`
with the `exec::_any::_any_receiver_ref` class template in
`any_sender_of.hpp`. The comments probably still need cleaning up, and
there's a `TODO` to pull the stuff in `any_sender_of.hpp` that's shared
between `exec::any_sender_of` and `exec::function` into a separate,
shared header.
---
 include/exec/function.hpp | 279 +++-----------------------------------
 1 file changed, 16 insertions(+), 263 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index e9eb06f89..a9a516d12 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -23,6 +23,9 @@
 #include "../stdexec/__detail/__scope.hpp"
 #include "../stdexec/__detail/__sender_concepts.hpp"
 
+// TODO: split this header into pieces
+#include "any_sender_of.hpp"
+
 #include <exception>
 #include <memory>
 #include <new>
@@ -76,6 +79,7 @@ namespace experimental::execution
 
   inline constexpr get_frame_allocator_t get_frame_allocator{};
 
+#if 0
   namespace _qry_detail
   {
     template <class Sig>
@@ -84,186 +88,12 @@ namespace experimental::execution
     template <class Return, class Query, class... Args, bool NoThrow>
     inline constexpr bool is_query_function_v<Return(Query, Args...) noexcept(NoThrow)> = true;
   }  // namespace _qry_detail
-
-  // a "type list" for bundling together function type representing queries to support in
-  // a type-erased environment. All of the types in Queries... must be (possibly noexcept)
-  // function types. For example:
-  //
-  //   queries<
-  //     std::execution::inline_stop_token(std::execution::get_stop_token_t) noexcept,
-  //     std::pmr::polymorphic_allocator<std::byte>(std::execution::get_allocator_t)
-  //   >
-  template <class... Queries>
-    requires(_qry_detail::is_query_function_v<Queries> && ...)
-  struct queries
-  {};
+#endif
 
   namespace _func
   {
     using namespace STDEXEC;
 
-    // a recursively-defined type with a vtable containing one virtual function for
-    // each query in Queries...
-    //
-    // the base template is an empty class, representing the empty set of queries.
-    template <class... Queries>
-    struct _env_of_queries
-    {};
-
-    // a special case in the recursion: when there is only one query in the pack, there's
-    // no base implementation of query to put in the using statement
-    template <class Return, class Query, class... Args, bool NoThrow>
-    struct _env_of_queries<Return(Query, Args...) noexcept(NoThrow)>
-    {
-      virtual Return query(Query query, Args &&...args) const noexcept(NoThrow) = 0;
-    };
-
-    // the recursive case that declares the named query as a pure virtual member function
-    // and inherits the rest of the required queries through inheritance
-    template <class Return, class Query, class... Args, bool NoThrow, class... Queries>
-    struct _env_of_queries<Return(Query, Args...) noexcept(NoThrow), Queries...>
-      : private _env_of_queries<Queries...>
-    {
-      _env_of_queries() = default;
-
-      _env_of_queries(_env_of_queries &&) = delete;
-
-      using _env_of_queries<Queries...>::query;
-
-      virtual Return query(Query query, Args &&...args) const noexcept(NoThrow) = 0;
-
-     protected:
-      ~_env_of_queries() = default;
-    };
-
-    // an environment type that delegates query to an _env_of_queries<Queries...> so that the
-    // environment type that we traffic in is cheaply copyable
-    template <class... Queries>
-    struct _delegate_env
-    {
-      using delegate_t = _env_of_queries<Queries...>;
-
-      explicit _delegate_env(delegate_t const &delegate) noexcept
-        : delegate_(std::addressof(delegate))
-      {}
-
-      template <class Query, class... Args>
-        requires __queryable_with<delegate_t, Query, Args...>
-      constexpr auto query(Query, Args &&...args) const
-        noexcept(__nothrow_queryable_with<delegate_t, Query, Args...>)
-          -> __query_result_t<delegate_t, Query, Args...>
-      {
-        return __query<Query>()(*delegate_, std::forward<Args>(args)...);
-      }
-
-     private:
-      delegate_t const *delegate_;
-    };
-
-    // in the base case, there's no need to store a pointer
-    template <>
-    struct _delegate_env<>
-    {
-      using delegate_t = _env_of_queries<>;
-
-      explicit _delegate_env(delegate_t const &) noexcept {}
-    };
-
-    template <class Sig>
-    struct _virt_completion;
-
-    // a vtable entry representing a receiver completion function; CPO should be a completion
-    // function (e.g. set_Value_t), and Args... is the expected argument list.
-    template <class CPO, class... Args>
-    struct _virt_completion<CPO(Args...)>
-    {
-      constexpr _virt_completion() = default;
-
-      _virt_completion(_virt_completion &&) = delete;
-
-      constexpr virtual void complete(CPO, Args &&...) noexcept = 0;
-
-     protected:
-      constexpr ~_virt_completion() = default;
-    };
-
-    template <class Sigs, class... Queries>
-    struct _virt_completions;
-
-    // a class template that bundles together a pure virtual completion function for each
-    // of the specified completion functions, and provides an implementation of get_env
-    template <class... Sigs, class... Queries>
-    struct _virt_completions<completion_signatures<Sigs...>, Queries...>
-      : _virt_completion<Sigs>...
-      , _env_of_queries<Queries...>
-    {
-      constexpr _virt_completions() = default;
-
-      _virt_completions(_virt_completions &&) = delete;
-
-      // this will complain if sizeof...(Sigs) == 0, but a sender with no completions
-      // isn't super useful...
-      using _virt_completion<Sigs>::complete...;
-
-      constexpr _delegate_env<Queries...> get_env() const noexcept
-      {
-        return _delegate_env<Queries...>(*this);
-      }
-
-     protected:
-      constexpr ~_virt_completions() = default;
-    };
-
-    template <class Sigs, class... Queries>
-    class _func_rcvr;
-
-    // a type-erased receiver expecting to be completed by one of the completions specified
-    // in Sigs..., and providing an environment that supports the queries specified in
-    // Queries...
-    //
-    // this is the receiver type that is passed into the sender being type-erased by a
-    // function<...>, and it forwards completions to the concrete receiver through the
-    // internal completer_ pointer
-    template <class... Sigs, class... Queries>
-    class _func_rcvr<completion_signatures<Sigs...>, Queries...>
-    {
-      using completer_t = _virt_completions<completion_signatures<Sigs...>, Queries...>;
-
-      completer_t *completer_;
-
-     public:
-      using receiver_concept = receiver_tag;
-
-      constexpr explicit _func_rcvr(completer_t &completer) noexcept
-        : completer_(std::addressof(completer))
-      {}
-
-      template <class Error>
-      constexpr void set_error(Error &&err) && noexcept
-        requires requires { completer_->complete(set_error_t{}, std::forward<Error>(err)); }
-      {
-        completer_->complete(set_error_t{}, std::forward<Error>(err));
-      }
-
-      constexpr void set_stopped() && noexcept
-        requires requires { completer_->complete(set_stopped_t{}); }
-      {
-        completer_->complete(set_stopped_t{});
-      }
-
-      template <class... Values>
-      constexpr void set_value(Values &&...values) && noexcept
-        requires requires { completer_->complete(set_value_t{}, std::forward<Values>(values)...); }
-      {
-        completer_->complete(set_value_t{}, std::forward<Values>(values)...);
-      }
-
-      constexpr auto get_env() const noexcept -> _delegate_env<Queries...>
-      {
-        return STDEXEC::get_env(*completer_);
-      }
-    };
-
     // the type-erased operation state type that supports starting and destruction
     struct _base_op
     {
@@ -316,72 +146,6 @@ namespace experimental::execution
       Allocator alloc_;
     };
 
-    // a recursive implementation of Base, which is expected to inherit from
-    // _virt_completions
-    template <class Base, class Derived, class... Sigs>
-    struct _func_op_completion;
-
-    // the base case of the recursive implementation; all subclasses of this type have,
-    // together, overridden all the virtual functions in Base so now we just need to
-    // inherit from Base to ensure those virtual functions exist to be overridden
-    template <class Base, class Derived>
-    struct _func_op_completion<Base, Derived> : Base
-    {};
-
-    // the recursive case, which implements a single overload of complete and delegates
-    // the implementation of all remaining overloads to the base class
-    template <class Base, class Derived, class CPO, class... Args, class... Sigs>
-    struct _func_op_completion<Base, Derived, CPO(Args...), Sigs...>
-      : _func_op_completion<Base, Derived, Sigs...>
-    {
-      void complete(CPO, Args &&...args) noexcept final
-      {
-        // This seems like it ought to be true, but it fails...
-        //
-        // Some testing shows it's being evaluated when Derived is incomplete
-        // during constraint satisfaction testing.
-        //
-        // static_assert(std::derived_from<_func_op_completion, Derived>);
-        //
-        // Consider: what if _func_op_completion<Base, Derived> (i.e. the base case of
-        //           this recursive class hierarchy) owned the receiver? We could avoid
-        //           CRTP and just use this->rcvr_, maybe.
-        auto &rcvr = static_cast<Derived *>(this)->rcvr_;
-        CPO{}(std::move(rcvr), std::forward<Args>(args)...);
-      }
-    };
-
-    // a recursive implementation of all the queries in Queries...
-    template <class Base, class Derived, class... Queries>
-    struct _func_op_queries;
-
-    // the base case of the recursive implementation; there are no more queries to
-    // implement so just inherit from Base
-    template <class Base, class Derived>
-    struct _func_op_queries<Base, Derived> : Base
-    {};
-
-    // the recursive case, which implements a single query overload and delegates the
-    // implementation of the remaining overloads to the base class
-    template <class Base,
-              class Derived,
-              class Return,
-              class Query,
-              class... Args,
-              bool NoThrow,
-              class... Queries>
-    struct _func_op_queries<Base, Derived, Return(Query, Args...) noexcept(NoThrow), Queries...>
-      : _func_op_queries<Base, Derived, Queries...>
-    {
-      Return query(Query, Args &&...args) const noexcept(NoThrow) final
-      {
-        // the idea of storing the receiver in the base class could help here, too, but
-        // we'd need to be careful about which class template is actually the base class
-        auto const &rcvr = static_cast<Derived const *>(this)->rcvr_;
-        return __query<Query>()(STDEXEC::get_env(rcvr), std::forward<Args>(args)...);
-      }
-    };
-
     template <class Receiver, class Sigs, class... Queries>
     class _func_op;
 
@@ -391,12 +155,6 @@ namespace experimental::execution
     // to a _func_rcvr
     template <class Receiver, class... Sigs, class... Queries>
     class _func_op<Receiver, completion_signatures<Sigs...>, Queries...>
-      : _func_op_completion<
-          _func_op_queries<_virt_completions<completion_signatures<Sigs...>, Queries...>,
-                           _func_op<Receiver, completion_signatures<Sigs...>, Queries...>,
-                           Queries...>,
-          _func_op<Receiver, completion_signatures<Sigs...>, Queries...>,
-          Sigs...>
     {
       // rcvr_ has to be initialized before op_ because our implementation of get_env
       // is empirically accessed during our constructor and depends on rcvr_ being initialized
@@ -407,13 +165,8 @@ namespace experimental::execution
       // a user-provided frame allocator
       std::unique_ptr<_base_op> op_;
 
-      // these friend declaratiosn allow our CRTP base classes to access rcvr_; they could
-      // disappear if we moved ownership of rcvr_ into the base class object
-      template <class, class, class...>
-      friend struct _func_op_completion;
-
-      template <class, class, class...>
-      friend struct _func_op_queries;
+      using _receiver_t =
+        ::exec::_any::_any_receiver_ref<completion_signatures<Sigs...>, queries<Queries...>>;
 
      public:
       using operation_state_concept = operation_state_tag;
@@ -421,7 +174,7 @@ namespace experimental::execution
       template <class Factory>
       constexpr _func_op(Receiver rcvr, Factory factory)
         : rcvr_(std::move(rcvr))
-        , op_(factory(_func_rcvr<completion_signatures<Sigs...>, Queries...>(*this)))
+        , op_(factory(_receiver_t(rcvr_)))
       {}
 
       _func_op(_func_op &&) = delete;
@@ -470,10 +223,12 @@ namespace experimental::execution
     template <class SndrCncpt, class... Args, class... Sigs, class... Queries>
     class _func_impl<SndrCncpt(Args...), completion_signatures<Sigs...>, queries<Queries...>>
     {
+      using _receiver_t =
+        ::exec::_any::_any_receiver_ref<completion_signatures<Sigs...>, queries<Queries...>>;
+
       // the type-erased sender factory that, when called, constructs the erased sender from
       // args_ and connects the resulting sender to the provided receiver
-      std::unique_ptr<_base_op> (*factory_)(_func_rcvr<completion_signatures<Sigs...>, Queries...>,
-                                            Args &&...);
+      std::unique_ptr<_base_op> (*factory_)(_receiver_t, Args &&...);
       STDEXEC_ATTRIBUTE(no_unique_address)
       std::tuple<Args...> args_;
 
@@ -486,16 +241,14 @@ namespace experimental::execution
         requires STDEXEC::__not_decays_to<Factory, _func_impl>  //
               && STDEXEC::__std::constructible_from<Factory>    //
               && STDEXEC::__callable<Factory, Args...>
-              && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>,
-                                    _func_rcvr<completion_signatures<Sigs...>, Queries...>>
+              && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>, _receiver_t>
       constexpr explicit _func_impl(Args &&...args, Factory &&factory)
         noexcept(STDEXEC::__nothrow_move_constructible<Args...>)
         : args_(std::forward<Args>(args)...)
       {
-        using sender_t   = std::invoke_result_t<Factory, Args...>;
-        using receiver_t = _func_rcvr<completion_signatures<Sigs...>, Queries...>;
+        using sender_t = std::invoke_result_t<Factory, Args...>;
 
-        factory_ = [](receiver_t rcvr, Args &&...args) -> std::unique_ptr<_base_op>
+        factory_ = [](_receiver_t rcvr, Args &&...args) -> std::unique_ptr<_base_op>
         {
           // the type of the allocator provided by the receiver's environment
           using alloc_t = decltype(choose_frame_allocator(get_env(rcvr)));
@@ -505,7 +258,7 @@ namespace experimental::execution
 
           // the type of operation we'll ultimately allocate, which depends on the type of
           // the allocator we're using
-          using op_t = _derived_op<sender_t, receiver_t, typename traits_t::allocator_type>;
+          using op_t = _derived_op<sender_t, _receiver_t, typename traits_t::allocator_type>;
 
           // finally, the allocator traits for an allocator that can allocate an op_t
           using traits = traits_t::template rebind_traits<op_t>;

From e0818b32ab1ba38f3ecaf7dee13aaa376132a160 Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Tue, 28 Apr 2026 21:26:19 -0700
Subject: [PATCH 19/20] Clean up the comments

Update comments to match the new implementation.
---
 include/exec/function.hpp | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index a9a516d12..679979198 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -107,8 +107,8 @@ namespace experimental::execution
     };
 
     // the operation state resulting from connecting a sender being erased by a function<...>
-    // with a _func_rcvr<...>; inherits from _base_op, and provides a class-specific override
-    // of operator delete that invokes the allocator deallocation protocol
+    // with an _any::_any_receiver_ref<...>; inherits from _base_op, and provides a
+    // class-specific override of operator delete that invokes the allocator deallocation protocol
     template <class Sender, class Receiver, class Allocator>
     struct _derived_op : _base_op
     {
@@ -149,10 +149,10 @@ namespace experimental::execution
     template <class Receiver, class Sigs, class... Queries>
     class _func_op;
 
-    // the concrete operation state resulting from connecting a function<...> to a concrete
-    // receiver of type Receiver. this type manages a dynamically-allocated _derived_op instance,
+    // The concrete operation state resulting from connecting a function<...> to a concrete
+    // receiver of type Receiver. This type manages a dynamically-allocated _derived_op instance,
     // which is the type-erased operation state resulting from connecting the type-erased sender
-    // to a _func_rcvr
+    // to an _any::_any_receiver_ref with the given completion signatures and queries.
     template <class Receiver, class... Sigs, class... Queries>
     class _func_op<Receiver, completion_signatures<Sigs...>, Queries...>
     {
@@ -388,7 +388,7 @@ namespace experimental::execution
   // should this require STDEXEC::__not_same_as<Return, STDEXEC::sender_tag>?
   //
   // you *could* write STDEXEC::just(STDEXEC::sender_tag{}), but it seems more likely
-  // that invokign this specialization with Return set to sender_tag is a bug...
+  // that invoking this specialization with Return set to sender_tag is a bug...
   //
   // the same question applies to all the specializations below that take explicit
   // completion signatures

From b65e281158b96a7ceeeccbdd7ada36cfbd429f7a Mon Sep 17 00:00:00 2001
From: Ian Petersen <ispeters@gmail.com>
Date: Tue, 28 Apr 2026 21:31:21 -0700
Subject: [PATCH 20/20] Stop deducing noexcept

Take code review feedback and replace attempts to deduce a function
type's `noexcept` clause with explicit partial specializations for both
the throwing and non-throwing cases.
---
 include/exec/function.hpp | 49 +++++++++++++++++++++++++++++++--------
 1 file changed, 39 insertions(+), 10 deletions(-)

diff --git a/include/exec/function.hpp b/include/exec/function.hpp
index 679979198..e71630638 100644
--- a/include/exec/function.hpp
+++ b/include/exec/function.hpp
@@ -85,8 +85,11 @@ namespace experimental::execution
     template <class Sig>
     inline constexpr bool is_query_function_v = false;
 
-    template <class Return, class Query, class... Args, bool NoThrow>
-    inline constexpr bool is_query_function_v<Return(Query, Args...) noexcept(NoThrow)> = true;
+    template <class Return, class Query, class... Args>
+    inline constexpr bool is_query_function_v<Return(Query, Args...)> = true;
+
+    template <class Return, class Query, class... Args>
+    inline constexpr bool is_query_function_v<Return(Query, Args...) noexcept> = true;
   }  // namespace _qry_detail
 #endif
 
@@ -384,7 +387,7 @@ namespace experimental::execution
   template <class...>
   struct function;
 
-  template <class Return, class... Args, bool NoThrow>
+  template <class Return, class... Args>
   // should this require STDEXEC::__not_same_as<Return, STDEXEC::sender_tag>?
   //
   // you *could* write STDEXEC::just(STDEXEC::sender_tag{}), but it seems more likely
@@ -392,13 +395,26 @@ namespace experimental::execution
   //
   // the same question applies to all the specializations below that take explicit
   // completion signatures
-  struct function<Return(Args...) noexcept(NoThrow)>
+  struct function<Return(Args...)>
+    : _func::_func_impl<STDEXEC::sender_tag(Args...),
+                        _func::_sigs_from_t<Return(Args...)>,
+                        queries<>>
+  {
+    using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
+                                   _func::_sigs_from_t<Return(Args...)>,
+                                   queries<>>;
+
+    using base::base;
+  };
+
+  template <class Return, class... Args>
+  struct function<Return(Args...) noexcept>
     : _func::_func_impl<STDEXEC::sender_tag(Args...),
-                        _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
+                        _func::_sigs_from_t<Return(Args...) noexcept>,
                         queries<>>
   {
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
-                                   _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
+                                   _func::_sigs_from_t<Return(Args...) noexcept>,
                                    queries<>>;
 
     using base::base;
@@ -414,14 +430,27 @@ namespace experimental::execution
     using base::base;
   };
 
-  template <class Return, class... Args, bool NoThrow, class... Queries>
-  struct function<Return(Args...) noexcept(NoThrow), queries<Queries...>>
+  template <class Return, class... Args, class... Queries>
+  struct function<Return(Args...), queries<Queries...>>
+    : _func::_func_impl<STDEXEC::sender_tag(Args...),
+                        _func::_sigs_from_t<Return(Args...)>,
+                        queries<Queries...>>
+  {
+    using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
+                                   _func::_sigs_from_t<Return(Args...)>,
+                                   queries<Queries...>>;
+
+    using base::base;
+  };
+
+  template <class Return, class... Args, class... Queries>
+  struct function<Return(Args...) noexcept, queries<Queries...>>
     : _func::_func_impl<STDEXEC::sender_tag(Args...),
-                        _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
+                        _func::_sigs_from_t<Return(Args...) noexcept>,
                         queries<Queries...>>
   {
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
-                                   _func::_sigs_from_t<Return(Args...) noexcept(NoThrow)>,
+                                   _func::_sigs_from_t<Return(Args...) noexcept>,
                                    queries<Queries...>>;
 
     using base::base;