Specialization of 'std::formatter<winrt::Windows::Foundation::IStringable, wchar_t>' breaks with conformant compilers

[dunhor]

E.g. when building with Clang 13 & the standard conformance flags, I see the error:

include\winrt/impl/Windows.Foundation.1.h(245,68): error: function 'ToString' with deduced return type cannot be used before it is defined
        return std::formatter<winrt::hstring, wchar_t>::format(obj.ToString(), fc);
                                                                   ^
include\winrt/impl/Windows.Foundation.0.h(864,34): note: 'ToString' declared here
        WINRT_IMPL_AUTO(hstring) ToString() const;

Seems that it's because the definition of consume_Windows_Foundation_IStringable<D>::ToString is defined in the "normal" winrt/Windows.Foundation.h header, but all of the winrt/impl/*.h files are included first, and one of these is the header trying to consume the function in question.

[sylveon]

Shouldn't it work if you include winrt/Windows.Foundation.h before making use of std::format?

[dunhor]

This break is caused simply by including winrt/Windows.Foundation.h. No use of std::format is necessary

[dunhor]

I'd assume that this is because IStringable::ToString is not dependent on any template parameter and so the compiler needs to know the return type, which it can't because the function is only declared at that time with an auto return type. The fix is to either not declare the function with an auto return type (probably not going to happen) or to define the specialization after the function's definition.

[dunhor]

E.g. see https://godbolt.org/z/v5b3Wf1Ea

[sylveon]

Moving it to winrt/Windows.Foundation.h would cause issues because specializations are generated for types which implement IStringable and they depend on the specialization at fault here, but the header for those doesn't necessarily include winrt/Windows.Foundation.h, while it does include winrt/impl/Windows.Foundation.1.h.

Two different ideas for a fix:

• Forward declare the specialization at the existing location, then implement formatter::format in winrt/Windows.Foundation.h, using the same auto trick as is used currently in other methods to avoid linker errors.
• Make formatter::format a template (and make obj depend on it), to prevent expansion until use.

In both cases, users will still need to include winrt/Windows.Foundation.h to use std::format, but this isn't anything new, they already have to do that to e.g. await IAsyncOperation.