Should we make `invoke` `constexpr` in all modes?

[frederick-vs-ja]

MSVC STL (non-conformingly) makes std::invoke available in C++14 mode, and makes it constexpr since C++17 mode (#585).

Does it make sense to leave std::invoke non-constexpr in C++14 mode? IIUC only non-conforming programs that use std::invoke in C++14 mode will be affected if we make std::invoke unconditionally constexpr.

[StephanTLavavej]

We were really scared of constexpr regressions early on, but I agree that this would be a reasonable thing to do now.

[StephanTLavavej]

We talked about this at the weekly maintainer meeting and are in favor of this change.