Please see the 2018 thread below. I think this macro should be added to SD-6 and the IS, and I think it should also be updated to 201811L for the P1032R1 "Misc constexpr bits" changes that made the remaining char_traits members constexpr. All the other bits of P1032R1 have their own macro now instead of __cpp_lib_constexpr_misc, but not the char_traits bits.

Libstdc++ has been defining __cpp_lib_constexpr_char_traits to 201611 for years (and I'm about to update the value to 201811 in the relevant branches).

Any objection before I open an LWG issue?