In C++03 mode evaluating the SFINAE can cause a hard error due to access control violations. This is a problem because the SFINAE is evaluated as soon as the class is instantiated, and not later. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@276594 91177308-0d34-0410-b5e6-96231b3b80d8