P2548R6 `copyable_function`

[StephanTLavavej]

WG21-P2548R6 copyable_function
LWG-4255 move_only_function constructor should recognize empty copyable_functions

Feature-test macro:

#define __cpp_lib_copyable_function 202306L

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.

[AlexGuteniev]

I want to claim this for myself.

I think we should reuse move_only_function machinery as much as possible.

[AlexGuteniev]

Quick regression test command:

py tests\utils\stl-lit\stl-lit.py ..\..\tests\std\tests\GH_000140_adl_proof_construction ..\..\tests\std\tests\GH_000690_overaligned_function ..\..\tests\std\tests\GH_005315_destructor_tombstones ..\..\tests\std\tests\GH_005504_avoid_function_call_wrapping ..\..\tests\std\tests\P0288R9_move_only_function ..\..\tests\std\tests\P2255R2_invocation -v