The current draft of D1902R1 proposes adding the macro
__cpp_constexpr_members_defined for the resolution of Core Issue 1581: When
are constexpr members defined?
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1581

I took the name of the macro directly from the core issue title. This macro
will be important for implementing certain library functionality (e.g.
constexpr invoke), but there was concern expressed in Evolution as to the
specific choice of the name of this macro. Richard in session suggested
that the name should be something like __cpp_constexpr_decltype instead,
given that that's what the issue's resolution is kind of about.

Does anybody having any naming preference for this macro?

Thanks,

Barry