Acceptable format strings

[matt77hias]

Any design reason why some of the below conversions do not work?

#include <fmt/core.h>
#include <fmt/xchar.h>

using FormatString     = fmt::basic_format_string< char,    int >;
using FormatWideString = fmt::basic_format_string< wchar_t, int >;

auto main() -> int
{
    [[maybe_unused]] auto f1 = fmt::format(FormatString    {  "{}" }, 1);
    [[maybe_unused]] auto f2 = fmt::format(FormatWideString{ L"{}" }, 1);// ERROR

    [[maybe_unused]] auto f3 = fmt::format( "{}", 1);
    [[maybe_unused]] auto f4 = fmt::format(L"{}", 1);

    [[maybe_unused]] auto f5 = fmt::format(fmt::basic_string_view< char >{    FormatString    {  "{}" } }, 1);// ERROR
    [[maybe_unused]] auto f6 = fmt::format(fmt::basic_string_view< wchar_t >{ FormatWideString{ L"{}" } }, 1);

    return 0;
}

https://godbolt.org/z/PYbcbWbaf

[vitaut]

The last two should not work because conversion from a format string to a string view is not constexpr. Compile-time checks are not fully implemented for wchar_t which is why the last one doesn't give an error. Technically they could be made to compile but it would introduce duplicate checks which is undesirable.

This on the other hand shouldn't give an error:

    [[maybe_unused]] auto f2 = fmt::format(FormatWideString{ L"{}" }, 1);// ERROR

Again this is because of compile-time checks not fully implemented for wchar_t.

Incorrect error/non-error cases should be fixed in e724bbe. Thanks for reporting.

[matt77hias]

Works for gcc. MSVC seems to have a bug: https://godbolt.org/z/dv7bz7hb8