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

[Casey Carter via Phabricator via cfe-commits]

CaseyCarter created this revision.
CaseyCarter added reviewers: mclow.lists, EricWF.

I'm breaking this up into independent bite-sized ~500 line reviews instead of a single 5200 line monster in hopes that we can turnaround changes quickly.

This first piece implements the new bullet for `common_type`, and all of `common_reference` and `basic_common_reference`. I've conservatively hidden everything behind `_LIBCPP_STD_VER > 17`, although it could be made available in C++11 mode with some edits.


https://reviews.llvm.org/D49118

Files:
  include/type_traits
  test/std/utilities/meta/meta.trans/meta.trans.other/common_reference.pass.cpp
  test/std/utilities/meta/meta.trans/meta.trans.other/common_type.pass.cpp

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