Provide case_() as a domain-specific alternative to switch_expression::append()

[ipapadop]

Motivation

switch_expression::append() expands a switch with additional case expressions:

auto expanded = ex.append(deferred::case_(11, body));

The generic name append does not describe what may be appended. A member named case_ would make the expression read more like the C++ construct it models:

auto expanded = ex.case_(11, body);

Proposed API

Consider either accepting a label and body directly:

template<typename Label, typename Body>
auto case_(Label&& label, Body&& body) const&;

template<typename Label, typename Body>
auto case_(Label&& label, Body&& body) &&;

or accepting already-constructed case expressions:

auto expanded = ex.case_(deferred::case_(11, body));

The direct label/body form is more concise and avoids the visually repetitive .case_(deferred::case_(...)).

Design considerations

• Existing cases must retain precedence over newly added cases.
• The operation should return a new flattened switch_expression, matching current append() value semantics.
• The const& overload should copy owned state; the && overload should move it.
• Move-only labels and bodies must work through the rvalue overload.
• Result types must be recomputed across the default, existing cases, and added case.
• Decide whether append() remains as an alias, is deprecated, or is eventually replaced.
• The overload must not introduce ambiguity with the namespace-level deferred::case_() factory.

Compatibility

Adding case_() alongside append() is backward compatible. Removing or renaming append() should be deferred to a major release.

Acceptance criteria

• A switch can be expanded using member case_().
• Existing-case, added-case, and default behavior are covered.
• Duplicate labels preserve existing-case precedence.
• Heterogeneous results are deduced correctly.
• Const-lvalue and move-only rvalue expansion are tested.
• The relationship between case_() and append() is documented.