Enable /await opt-out so users can manually enable /await:strict.

[jlaanstra]

/await:strict is binary compatible with c++20 coroutines. Instead of adding new logic to the C++/WinRT build targets, allow opt-out of the current default so users can set this manually.

For details, see: https://docs.microsoft.com/en-us/cpp/build/reference/await-enable-coroutine-support?view=msvc-160 and https://devblogs.microsoft.com/cppblog/cpp20-coroutine-improvements-in-visual-studio-2019-version-16-11/

Fixes #1014

[oldnewthing]

The downside is that it means two libraries both compiled for C++17 with the same version of cppwinrt.exe but different versions of the NuGet package (or one compiled with the NuGet package and one without)[1] are now no longer ABI compatible. I think we need to make this opt-in rather than a change in default. (Or at least, if it's a change in default, a way to opt out.)

[1] Example: A module that contains both C++/WinRT code (in C++17 mode and /await:strict) and C+/CX code (which uses C++17 and plain /await). We have a lot of modules like this internally inside Windows.

[jlaanstra]

My understanding is that CX code can also use /await:strict, My primary motivation for this is that without it you can't mix a static C++/WinRT library compiled with stdcpp20 within a dll project that uses C++/CX and therefore always uses stdcpp17.

[jlaanstra]

I am a fan of having opt-outs though so let me add that.

[sylveon]

Yeah, I am generally not a fan of things like NuGet packages forcing command line arguments on the project - if I was involved in the original design, I would have had the templates include /await and never touch the command line args in the NuGet package, as it now causes issues such as this (and the fact you couldn't compile C++20 until cppwinrt was updated, as well)

[jlaanstra]

Yeah, I am generally not a fan of things like NuGet packages forcing command line arguments on the project - if I was involved in the original design, I would have had the templates include /await and never touch the command line args in the NuGet package, as it now causes issues such as this (and the fact you couldn't compile C++20 until cppwinrt was updated, as well)

Agreed, instead of adding /await:strict, I'm instead adding the option to opt-out of the current /await, so project maintainers can disable it and pass their own option.

[kennykerr]

Why not just remove this, or does this affect existing projects?

[sylveon]

Removing it entirely could affect projects which relied on the implicit addition of /await.

[kennykerr]

Makes sense, the logic is just throwing me but then msbuild has never made sense to me.