Forward declare is_string<FILE*> specializations

[riccozzz]

Not sure if this is the ideal or correct solution, but this is how I got around issue #1085

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. I think the root cause is that we allow anything as a character type, so a better solution would be to only provide overloads of to_string_view for const char* and const wchar_t* instead of

fmt/include/fmt/core.h

Line 500 in b742f62

inline basic_string_view<Char> to_string_view(const Char* s) {

and remove those weird specializations in

fmt/include/fmt/color.h

Line 525 in b742f62

template <> struct is_string<std::FILE*> : std::false_type {};

.

[riccozzz]

only provide overloads of to_string_view for const char* and const wchar_t*

Correct me if I'm wrong, but wouldn't you at least need char16_t and char32_t as well just to pass unit tests? (and nullptr_t would become ambiguous). Also, wouldn't this be a breaking change for anyone doing something with weird char types?

[vitaut]

Hmm, you are right. It's not enough to handle char and wchar_t, we need some kind of is_char trait. Anyway, I'll merge your workaround for now to unbreak master. Thanks again!