Conditionally conform Chain2Sequence to MutableCollection

Guides/Chain.md

@@ -30,8 +30,8 @@ public func chain<S1, S2>(_ s1: S1, _ s2: S2) -> Chain2Sequence<S1, S2>
 ```
 
 The resulting `Chain2Sequence` type is a sequence, with conditional conformance
-to `Collection`, `BidirectionalCollection`, and `RandomAccessCollection` when
-both the first and second arguments conform.
+to `Collection`, `BidirectionalCollection`, `RandomAccessCollection`, and
+`MutableCollection` when both the first and second arguments conform.
 
 ### Naming
 

Sources/Algorithms/Chain.swift

@@ -14,11 +14,11 @@ public struct Chain2Sequence<Base1: Sequence, Base2: Sequence>
 where Base1.Element == Base2.Element {
   /// The first sequence in this chain.
   @usableFromInline
-  internal let base1: Base1
+  internal var base1: Base1
 
   /// The second sequence in this chain.
   @usableFromInline
-  internal let base2: Base2
+  internal var base2: Base2
 
   @inlinable
   internal init(base1: Base1, base2: Base2) {
@@ -292,6 +292,37 @@ where Base1: BidirectionalCollection, Base2: BidirectionalCollection {
 extension Chain2Sequence: RandomAccessCollection
 where Base1: RandomAccessCollection, Base2: RandomAccessCollection {}
 
+extension Chain2Sequence: MutableCollection
+where Base1: MutableCollection, Base2: MutableCollection {
+  @inlinable
+  public subscript(i: Index) -> Base1.Element {
+    get {
+      switch i.position {
+      case .first(let i):
+        return base1[i]
+      case .second(let i):
+        return base2[i]
+      }
+    }
+    set {
+      switch i.position {
+      case .first(let i):
+        base1[i] = newValue
+      case .second(let i):
+        base2[i] = newValue
+      }
+    }
+    _modify {
+      switch i.position {
+      case .first(let i):
+        yield &base1[i]
+      case .second(let i):
+        yield &base2[i]
+      }
+    }
+  }
+}
+
 //===----------------------------------------------------------------------===//
 // chain(_:_:)
 //===----------------------------------------------------------------------===//

Tests/SwiftAlgorithmsTests/ChainTests.swift

@@ -70,4 +70,21 @@ final class ChainTests: XCTestCase {
       XCTAssertNil(j)
     }
   }
+
+  func testChainMutableCollection() {
+    let a = [1, 2, 3]
+    let b = [4, 5, 6]
+    var c = chain(a, b)
+
+    c[c.startIndex] = 10
+    let secondStart = c.index(c.startIndex, offsetBy: 3)
+    c[secondStart] = 40
+
+    expectEqualSequences(c, [10, 2, 3, 40, 5, 6])
+
+    for i in c.indices {
+      c[i] *= 2
+    }
+    expectEqualSequences(c, [20, 4, 6, 80, 10, 12])
+  }
 }