[PATCH] D49118: [libc++] P0898R3 1 of 12: changes to common_type; implement common_reference

[Marshall Clow via Phabricator via cfe-commits]

mclow.lists added a comment.

>   I've conservatively hidden everything behind _LIBCPP_STD_VER > 17, although it could be made available in C++11 mode with some edits.

If we have a use for this in pre c++2a code, the libc++ convention would be to implement `__common_reference` that works most everywhere, and then have `common_reference` that only works > 17, but inherits from the ugly name.

That might be difficult in this case, because users can extend `common_reference`.


https://reviews.llvm.org/D49118