tuple-formatter doesn't check that the tuple-elements are formattable.

[jehelset]

tuple-formatter doesn't check that the tuple-elements are formattable:
https://github.com/fmtlib/fmt/blob/master/include/fmt/ranges.h#L285-L287

which makes is_formattable<std::tuple<T>>::value true:
https://godbolt.org/z/G3zsT4q4W

even when the T is not formattable, and trying to format std::tuple<T> will lead to a hard error.

[vitaut]

Thanks for reporting. A PR to add such check would be welcome!