Define co_await operators for C++20 coroutines

[kobykahane]

Fixes #764.

[kennykerr]

/azp run

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[kennykerr]

Thanks Koby, this looks reasonable but I'd prefer to change the definition in one place rathe than duplicate it. Perhaps you can experiment with updating the following? This needs to support both C++20 and C++17 with the Visual C++ /await flag.

cppwinrt/strings/base_macros.h

Lines 30 to 32 in 714c085

	#if defined(_RESUMABLE_FUNCTIONS_SUPPORTED) && !defined(__cpp_coroutines)
	#define __cpp_coroutines
	#endif

[kobykahane]

Thanks Koby, this looks reasonable but I'd prefer to change the definition in one place rathe than duplicate it. Perhaps you can experiment with updating the following? This needs to support both C++20 and C++17 with the Visual C++ /await flag.

cppwinrt/strings/base_macros.h

Lines 30 to 32 in 714c085

	#if defined(_RESUMABLE_FUNCTIONS_SUPPORTED) && !defined(__cpp_coroutines)
	#define __cpp_coroutines
	#endif

I assume you mean define a cppwinrt-specific macro here and use it, rather than define the Coroutines TS feature test macro in C++20 coroutines mode too, right? I'll update the PR accordingly.

[kennykerr]

Perhaps the base_macros.h would look something like this:

#if defined(_RESUMABLE_FUNCTIONS_SUPPORTED) && !defined(__cpp_lib_coroutine) 
#define __cpp_lib_coroutine 
#endif 

And then subsequent checks would simply test for the C++20 feature macro. I think that would work, although I haven't tested it.

[sylveon]

Couldn't that mislead other libraries which use the macro for feature detection as well?

[kennykerr]

Perhaps - then we should define a WINRT_IMPL_COROUTINES macro.

You also need to account for this:

cppwinrt/strings/base_includes.h

Lines 27 to 53 in ff7ab25

	#ifdef __cpp_lib_coroutine
	#include <coroutine>
	namespace winrt::impl
	{
	template <typename T = void>
	using coroutine_handle = std::coroutine_handle<T>;
	using suspend_always = std::suspend_always;
	using suspend_never = std::suspend_never;
	}
	#else
	#include <experimental/coroutine>
	namespace winrt::impl
	{
	template <typename T = void>
	using coroutine_handle = std::experimental::coroutine_handle<T>;
	using suspend_always = std::experimental::suspend_always;
	using suspend_never = std::experimental::suspend_never;
	}
	#endif

[kennykerr]

Of course, if we can fix base_macros.h to not define a feature macro and then update the rest, as Koby originally did, to rely only on those feature macros that would save introducing yet another macro.

[kennykerr]

/azp run

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[kobykahane]

@kennykerr Any additional comments?

[kennykerr]

That looks great - I've spun a build. If all looks good I'll merge this in - thanks!

[kennykerr]

Looks great - thanks Koby!