C++14 support is not enabled with msvc2015

[Predelnik]

_HAS_CXX14 is never defined by MSVC
Proof:
https://gcc.godbolt.org/z/5odXyR

Also you may check that Microsoft STL uses only _HAS_CXX17

So second half of check for C++14 version support is incorrect (never triggered):

#elif (defined(__cplusplus) && __cplusplus >= 201402L) || (defined(_HAS_CXX14) && _HAS_CXX14 == 1)

What it leads to - on older msvc's which was not defining __cplusplus properly (at least on msvc2015) C++14 is never treated as supported even though it is supported. Currently it only disables usage of transparent comparator.

Unfortunately it seems there is no correct way to check for C++14 support for msvc since lowest value of their language switch is also c++14 but more or less it was supported starting from VS2015 and since it is the lowest version library supports, maybe treating C++14 as always supported by msvc is the best option?

[nlohmann]

If this is a bug in MSVC, then I am afraid I can do nothing in this library. Any ideas?

[Predelnik]

I would not say it's a msvc bug per se, it's just that current check (_HAS_CXX14 etc.) doesn't mean anything when compiling with msvc. Basically, msvc starting from 2015 supports c++14 unconditionally with any compilation flags, previous versions probably do not support it.

So in my opinion considering that minimum supported msvc version of the library is 2015, just replacing

 || (defined(_HAS_CXX14) && _HAS_CXX14 == 1)

with

|| defined (_MSC_VER)

should be fine.

[stale]

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