Fix #2034

[dkavolis]

For some reason clang doesn't like a disabled constructor in std::is_convertible so conversion to format strings has to be replaced by conversion to equivalent string views as in fmt.

This time also tested with Clang 12.0.1 C++20. Might be a good idea to add C++20 to CI.

Fixes #2034

[boxerab]

@dkavolis thank you! this patch fixed errors I started getting from clang 12 and c++20 after updating to latest v1.x version.

[gabime]

Does it compile if SPDLOG_WCHAR_TO_UTF8_SUPPORT (and hence is_convertible_to_wformat_string ) are not defined ? Or does SFINAE allows it?

Anyway I think it is cleaner just to remove the all those is_convertible_to_wformat_string definitions completely (lines 134-147) and do something like:

template<class T>
struct is_convertible_to_any_format_string
    : std::integral_constant<bool, is_convertible_to_basic_format_string<T, char>::value || is_convertible_to_basic_format_string<T, wchar>::value>
{};

[dkavolis]

is_convertible_to_wformat_string is defined the same way as is_convertible_to_wstring_view but a quick search in the repo showed no uses of either outside of logger::log SFINAE check if msg is convertible to a string view or format string. I used is_convertible_to_wstring_view as an example for is_convertible_to_wformat_string since they are both very similar. Might be cleaner to remove both in this case.

[gabime]

yes. please remove them

[dkavolis]

Also removed the SFINAE check for string view conversion in logger::log since format strings already check for it.

[gabime]

Thanks @dkavolis

[gabime]

@dkavolis I added CI to test c++20 but it fails here: https://travis-ci.com/github/gabime/spdlog/jobs/530329470

Could you please take a look please? (I tried fixing with wrapping with fmt::runtime the problematic line but it didn't help)

[dkavolis]

Seems like an issue with clang, the error happens when using std::strlen at compile time

  FMT_CONSTEXPR_CHAR_TRAITS
  FMT_INLINE
  basic_string_view(const Char* s) : data_(s) {
    if (detail::const_check(std::is_same<Char, char>::value &&
                            !detail::is_constant_evaluated()))
      size_ = std::strlen(reinterpret_cast<const char*>(s));
    else
      size_ = std::char_traits<Char>::length(s);
  }

But the only way this can happen is if either __cpp_lib_is_constant_evaluated is undefined, std::is_constant_evaluated() returns false or clang tries to evaluate the discarded branch in constant expression, neither of which should happen.

Can it be due to the old libstdc++ version? GCC is only version 7.5.0 and __cpp_lib_is_constant_evaluated needs GCC 9.0.

[gabime]

So I will try replace the ci to gcc and see what happens

Update:
@dkavolis seems you are right. I added gcc-11 CI test and it passes (https://travis-ci.com/github/gabime/spdlog/jobs/530467284).