Improve error message when trying to format wide into narrow string

[vitaut]

Compiling the code

#include <string>
#include <fmt/format.h>

int main()
   {
   auto s1 = fmt::format("format with parameter {}", "string");
   auto s2 = fmt::format("format with parameter {}", L"wstring");
   }

with VS 2013 Update 4 gives the following error:

1>fmt/format.h(724): error C2248: 'fmt::internal::CharTraits<char>::convert' : cannot access private member declared in class 'fmt::internal::CharTraits<char>'
1>          fmt/format.h(457) : see declaration of 'fmt::internal::CharTraits<char>::convert'
1>          fmt/format.h(454) : see declaration of 'fmt::internal::CharTraits<char>'
1>          fmt/format.h(723) : while compiling class template member function ‘void fmt::internal::MakeValue<char>::set_string(fmt::WStringRef)'
1>          fmt/format.h(800) : see reference to function template instantiation 'void fmt::internal::MakeValue<char>::set_string(fmt::WStringRef)' being compiled
1>          fmt/format.h(2485) : see reference to class template instantiation 'fmt::internal::MakeValue<char>' being compiled
1>          Test.cpp(6) : see reference to function template instantiation 'std::string fmt::format<const char[7]>(fmt::StringRef,const char (&)[7])' being compiled

which is rather confusing because the error points to the first format statement, not the second. This is due to a low quality of MSVC diagnostics as both GCC and Clang point exactly to the source of error, but would be nice to do something about it anyway.

[vitaut]

Oh my, MSVC is horrible.

[vitaut]

There are at least two options:

1. Make MakeValue ctors taking wide strings private with enable_if or similar if Char is char. This will require having two wide string ctors, one private and one public.
2. Generate an error in MakeValue ctor taking wide string when Char is char.

[vitaut]

Another thing to consider is that MakeValue's ctors should accept (or forbid) anything convertible to wide strings.

[vitaut]

Fixed in f014d32.

[vitaut]

Check if the same needs to be done for wide chars.

[vitaut]

Yes, it does.

[vitaut]

wchar_t is done in ed49014, but need to check writers as well.

[vitaut]

Fixed for writers too.