This patch fixes llvm.org/PR32097 by using the __is_abstract builtin type-trait instead of the previous library-only implementation. All supported compilers provide this trait. I've tested as far back as Clang 3.2, GCC 4.6 and MSVC trunk. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@296561 91177308-0d34-0410-b5e6-96231b3b80d8