Explain why invoke is implemented with a macro (#368)

stl/inc/type_traits

@@ -1476,6 +1476,11 @@ _NODISCARD constexpr conditional_t<!is_nothrow_move_constructible_v<_Ty> && is_c
 template <class _Ty>
 class reference_wrapper;
 
+// std::invoke isn't constexpr in C++17, and implementers are forbidden from "strengthening" constexpr (WG21-N4842
+// [constexpr.functions]/1), yet both std::apply and std::visit are required to be constexpr and have invoke-like
+// behavior. We solve this by using a macro to stamp out both public non-constexpr and internal constexpr
+// implementations. TRANSITION, P1065R2
+
 #define _CONCATX(x, y) x##y
 #define _CONCAT(x, y)  _CONCATX(x, y)