VC's current lack of an AST makes constexpr and other major features (especially variadic templates) very difficult. As the compiler devs have explained to me,

VC's current lack of an AST makes constexpr and other major features (especially variadic templates) very difficult. As the compiler devs have explained to me, this was a clever way to save space and time decades ago when C++ was simpler to compile. Features like constexpr and variadic templates make C++ easier to use, but they demand much more from the compiler's data structures.