Please see this CE snippet: https://godbolt.org/g/CVWCax

As a solution, I propose either:

• adding a macro FMT_HAS_CXX11, that checks for __cplusplus and/or compiler versions.
• defining FMT_HAS_GXX_CXX11 as a non-zero value on clang as well

I'm not proposing:

• defining FMT_USE_TRAILING_RETURN to zero instead of not defining it at all,

as this would lead to trailing returns to not be used on clang.

Once we reach a solution, I can submit a PR.