Creating a function pointer with proper parameters pointing to std::next() or std::prev() should work.
This change moves the invented paramater for enable_if over to the return type to resolve this QoI issue.
Patch by Jason Liu.
Differential Revision: https://reviews.llvm.org/D34649
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@308932 91177308-0d34-0410-b5e6-96231b3b80d8
