Summary: This is used to constrain `variant`'s converting constructor correctly. Reviewers: EricWF, mclow.lists Reviewed By: EricWF, mclow.lists Differential Revision: https://reviews.llvm.org/D34111 git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305370 91177308-0d34-0410-b5e6-96231b3b80d8