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

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`.