Error compiling fmtlib 6.2.0 with Intel's Compiler (2020 - v19.1) in C++14 or C++17 modes

Hello! I've been using fmtlib (6.2.0) successfully in C++17 mode with clang, gcc, and MSVC. However, I just tried using Intel's compilers (v19.1) and ran into significant errors.

If I try to build the test code from the fmtlib repo as is, the code builds fine in C++11 mode.

When I switch to either C++14 or C++17 mode, I get the errors (I just set CMAKE_CXX_STANDARD to 14 or 17) in this gist: https://gist.github.com/InnerPortal/db52fcebf16bb49fe50303ace24a9c24

On further investigation, looks like the problem is with use of (relaxed) constexpr. It looks like Intel supports extended constexpr in v19, but for some reason fails to build when FMT_USE_CONSTEXPR is set to on and FMT_CONSTEXPR is set to constexpr.

Changing the test for FMT_USE_CONSTEXPR in core.h to exclude the Intel compilers allows the code to compile. Works for me, for now, but I can't tell if there's a better way to solve this problem.

Thanks!

[vitaut]

Changing the test for FMT_USE_CONSTEXPR in core.h to exclude the Intel compilers allows the code to compile.

I think it's a reasonable workaround. Could you submit a PR?

[vitaut]

Should be worked around in 6e8d7e2.