VS2017 compiler suggests using constexpr if

[DragonOsman]

I'm making a bug report, but it's not really a bug. It's something between a bug and a feature request. Basically, the MSVC compiler (latest version in VS2017) is suggesting to use constexpr if in the file single_include\nlohmann\json,hpp on line 1966. The warning is:

warning C4127: conditional expression is constant
note: consider using 'if constexpr' statement instead

Also, on single_include\nlohmann\json.hpp(1962) (`962 is the line number):

note: while compiling class template member function 'std::char_traits<char>::int_type nlohmann::detail::wide_string_input_adapter<std::u32string>::get_character(void) noexcept'

[nlohmann]

Thanks for the info! Unfortunately, I am not sure what to do with it, because we target C++11, and constexpr if are a C++17 extension IIRC. So the best I could do to silence this warning would be to decorate it with some macros. Not sure if this is worth it, though...

[jk-jeon]

VS2017 now properly supports feature testing macros, which others already have supported.
How about this?

#ifdef __cpp_if_constexpr
  if constexpr( ... )
#else
  if( ... )
#endif

[theodelrieu]

I would rather write a metafunction than introduce some ifdefs.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nlohmann]

Fixed by merging #1272.