Make format work with C++17 std::string_view

[feroldi]

Enable std::string_view to work with format when compiling with C++17.

Change log:

• fmt::StringRef is constructible from std::string_view.
• fmt::StringRef is convertible to std::string_view (only explicitly)[1].
• fmt::format accepts std::string_view as an argument for the argument list.

[1] This is open to debate. Should it be an implicit conversion, or should it not exist at all? The argument in favor of it is that std::string_view is now the common string reference, and all other string implementations could take advantage of it by providing a conversion to it. A counter-argument would be that fmtlib prefers fmt::StringRef as the common string reference.

[vitaut]

Looks great, thanks for the PR!

Should it be an implicit conversion, or should it not exist at all?

Conversion seems fine unless it causes some issues for existing code such as overload ambiguity. Eventually StringRef will be replace with std::string_view anyway.

[mridgewell]

Thanks for this, but it doesn't work in Visual Studio because the __cplusplus definition is out of date in Microsoft's compiler. I suggest either removing that check or adding a specific one for _MSC_VER.

[vitaut]

@mridgewell If you are interested in string_view support on MSVC, could you submit a PR adding a version check specific to this compiler to

#if FMT_HAS_INCLUDE(<string_view>) && __cplusplus > 201402L

?

[feroldi]

Removing the check for __cplusplus prevents GCC from compiling, even if you're not making use of string_view in your own code. There's no issue with Clang though, as it accepts <string_view> in any standard version. I'm not sure about MSVC.

[mridgewell]

I wouldn't mind, but looks like @mwinterb has already done the work, which solves my problem :)
Thanks!