🐛 Cannot call non-constexpr function in constexpr context

[jgopel]

Problem:

• gcc-8 gives the following error when compiling this function on all
  standards:

    test/std-format-test.cc: In member function 'constexpr auto std::formatter<S>::parse(std::format_parse_context&)':
    test/std-format-test.cc:112:17: error: call to non-'constexpr' function 'int isdigit(int)'
        if (!isdigit(c) || (++iter, get_char()) != '}')
         ~~~~~~~^~~

Solution:

• Write a constexpr version of isdigit for use in this function.

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

[foonathan]

I think a better solution would be to use '0' <= c <= '9' instead, which can keep it constexpr.

[vitaut]

Thanks