Implementation of N3760 which is planned to be voted into C++14's draft
tomorrow is complete.

There is a missing warning due to a serious issue with template
instantiation in Clang (and potentially in the core language).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191577 91177308-0d34-0410-b5e6-96231b3b80d8