Currently, the internal state class _Packaged_state used by packaged_task stores a function.

However, function requires the stored target object to be copy constructible while packaged_task doesn't, which rendered our implementation strategy non-conforming (reject-valid) for a long while, see #321.

#4946 partially fixed the non-conformance. However, the fix was imperfect:

• it introduced handling for move-only functors which function isn't supposed to support,
• it was broken when one specializes function for program-defined types, although I don't think anyone should do this, and
• when the functor's copy constructor is eligible but ill-formed, there's still hard error from it, which isn't supposed to happen for packaged_task.

In vNext where ABI breakage is available, we need to change the internal storage type. It seems that the implementation details of that type can be shared with move_only_function, but it doesn't seem possible to directly use move_only_function because move_only_function is only available since C++23.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.