We haven't had a __cpp_concepts since the TS specified its value as 201507 - which GCC defines. I've been making up values since then for various compilers which cmcstl2 supports to various degrees:
* clang-concepts defines __cpp_concepts to 201707L - the date of the Toronto merge - to indicate support for concepts as merged.
* MSVC uses 201811L - the date of the P1084 merge - to indicate that it supports the above plus P1084's changes to return-type-requirements. (Unfortunately MSVC does not yet support the changes in P1141 which were merged at the same meeting.)
I'd appreciate a bump for the changes in Cologne. P1452 seems significant enough to me, but regardless it would be nice to have a way to discriminate between what MSVC will support in 16.3 and C++20 concepts' final form.