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?
It could do, but it affects string_view equally.
I suppose we could say that the current values for constexpr_string and constexpr_string_view also assume support for constexpr in char_traits, so that implementations should not define the latest values until they've done the char_traits parts.
So we'd have three possible values for constexpr_string, 201611 for P0436, 201811 for P1032, and 201907 for P0980 (which is already there). And two values for constexpr_string_view, 201611 for P0436 and 201811 for P1032 (which is already there).
I can send a pull request to do that if you like.