Implement LWG-3720 Restrict the valid types of arg-id for width and precision in std-format-spec

[fsb4000]

Fixes #3425

[fsb4000]

std::format(L"{:*^{}}", 'a', '0');

'0' is type erasured as wchar_t, (_Arg == Char state)
and later it passes the condition _Standard_signed_or_unsigned_integer<_Ty> because wchar_t == unsigned short

I'm not sure what we should do with /Zc:wchar_t-

[frederick-vs-ja]

I think we can still tell char_t from standard signed/unsigned integer types on /Zc:wchar_t-, because signed/unsigned integer types are never type-erased as unsigned short (only int, unsigned int, long long, and unsigned long long would be the erased type).

The real problem, IMO, is that it's impossible to conform to both [format.arg]/6.1 and [format.arg]/6.4 when wchar_t is same as unsigned short.

[fsb4000]

Thank @frederick-vs-ja !

Ok, I got you, so this code is accepted with /Zc:wchar_t- (and it throws without /Zc:wchar_t-)

std::wstring s = std::format(L"{:*^{}}\n", 'a', L'0');

and the lwg issue has the sentence:

The following integral types are rejected by both libc++ and MSVC STL:

std::cout << std::format("{:*^{}}\n", 'a', L'0');

[StephanTLavavej]

This works because we have machinery that promotes tiny types before they're seen here. The test coverage for signed char is sufficient.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for the precise PR! 🎯 🎉 😸