Add file stream support for stylized text printing.

[Rakete1111]

This PR adds file stream support to color.h:

fmt::print(stderr, fmt::emphasis::bold, "bold error");

Fixes #944

[vitaut]

LGTM, but could you rebase and add a small unit test that exercises the new function? Thanks!

[vitaut]

What is this for?

[Rakete1111]

If I do fmt::print(stderr, fmt::emphasis::bold, "");, OR kicks in. It has a lot of candidates:

print(std::FILE *, const text_style &, format_str, args...); // this is what we want to get called
print(format_str, args...);

Normally, one would expect the first one to get called, but that's not the case. This is due to variadic templates being greedy. fmt::emphasis::bold is not a text_style and since stderr is a pointer to T, the second overload above is taken, which then fails with an assertion failure because std::FILE is not a valid character type.

I'm adding this so that fmt doesn't think that stderr is a valid format string - This never was an issue before because there were always exact matches.

[DanielaE]

Wouldn't it be much nicer to disregard all pointer types cv T * for which no specialization std::char_traits<T> exists? May be by detecting the existence of the std::char_traits<T>::char_type type member which requires only type-SFINAE and should work on all compilers.

[Rakete1111]

@DanielaE Didn't want to change this for such a patch, but yeah that's the better solution. I'll update my PR then :) thanks

[DanielaE]

In this case I'd rather split the PR into two: the first one laying the cv T * groundwork and then the second one actually addressing issue #944 taking advantage of the first PR. This looks cleaner to me but at the end of the day it's up to Victor.

[vitaut]

Yeah, it looks like std::char_traits is defined for everything including FILE =), so we'll have to rely on some mechanism within {fmt} to detect char types. to_string_view is indeed our current way to detect string types and therefore character types, so unless there are better ideas let's go with the current to_string_view-based solution but please add a comment briefly explaining what's going on.

[DanielaE]

😢 This sounds like a language or library defect. Characters are fundamental entities in the language and there are predicates to detect almost any kind of trait, but a well-defined predicate to detect the "character-ness" type trait seems to be missing. I've sifted through MSVC's library and found the catch-all primary template of std::char_traits which defines the char_type member to T. Interestingly enough, only in this non-character case the int_type is defined to long. But this cannot be relied on because the specification in the standard is weasel-wordy in the case of non-required specializations.

[vitaut]

One more thing: to_string_view(const std::FILE *) should probably go to fmt/color.h since it is only relevant for the print overloads there.

[Rakete1111]

@vitaut It doesn't seem to work when I move the overload to color.h - don't know why. So I changed it to two explicit specializations instead. Hope that's ok too :)

[vitaut]

I think specializations are even clearer.

[vitaut]

Merged, thanks a lot for another great PR!