Rearrange feature test macros

[cpplearner]

Fixes #2135.

Note: I noticed that cl defines __cpp_impl_coroutine when /await:strict is on, so I believe that there's no need to check defined(_DOWNLEVEL_COROUTINES_SUPPORTED). I also relocated the definition of __cpp_lib_coroutine to make it next to the other C++20 feature-test macro that's not guarded by _HAS_CXX20.

[CaseyCarter]

Note: I noticed that cl defines __cpp_impl_coroutine when /await:strict is on, so I believe that there's no need to check defined(_DOWNLEVEL_COROUTINES_SUPPORTED).

Confirmed. IIRC (maybe @joemmett remembers better) we waffled a bit on whether we wanted to define __cpp_impl_coroutine in sub-C++20 modes, but the final state of the compiler (from my reading) does seem to be that defined(_DOWNLEVEL_COROUTINES_SUPPORTED) is functionally determined by defined(__cpp_impl_coroutine) && _MSVC_LANG < 202002L.

When porting this change internally, we should add a commit to remove the _DOWNLEVEL_COROUTINES_SUPPORTED predefine from the compiler.

[joemmett]

Note: I noticed that cl defines __cpp_impl_coroutine when /await:strict is on, so I believe that there's no need to check defined(_DOWNLEVEL_COROUTINES_SUPPORTED).

Confirmed. IIRC (maybe @joemmett remembers better) we waffled a bit on whether we wanted to defined __cpp_impl_coroutine in sub-C++20 modes, but the final state of the compiler (from my reading) does seem to be that defined(_DOWNLEVEL_COROUTINES_SUPPORTED) is functionally determined by defined(__cpp_impl_coroutine) && _MSVC_LANG < 202002L.

Correct. Looking back at the original PR the first implementation sensed _DOWNLEVEL_COROUTINES_SUPPORT directly in <coroutine> without either the impl or lib feature test macro being present. This was later changed to the current implementation.

When porting this change internally, we should add a commit to remove the _DOWNLEVEL_COROUTINES_SUPPORTED predefine from the compiler.

We can do that, we just need to make sure nobody is depending on this define (a quick search didn't turn up anything).

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[CaseyCarter]

Thanks for sweeping up this mess! 🧹