Using any_object with any_scheduler

[ccotter]

I tried replacing any_unique_t with any_object_t in any_scheduler to take advantage of the small object optimization, but seem to be hitting some sort of recursion in instantiating the template. any_object instantiates a class invalid_obj which ultimately checks whether each CPO can be invoked. any_scheduler includes the _copy_as CPO which expects any_scheduler to be defined, but finds that any_scheduler is an incomplete type.

any_unique does not have the same invalid_obj class as any_object does.

In https://github.com/facebookexperimental/libunifex/compare/main...ccotter:libunifex:soo-any_scheduler?expand=1, clang and GCC-11 or above compile if I comment out the static_assert(can_be_type_erased_v<invalid_obj>). When the static_assert is added back, these compilers fail with the incomplete type error. GCC-10 fails to compile even with the static_assert commented out.

This godbolt link is also a replication of the error.

cc @lewissbaker