Pull in r283944 from upstream libc++ trunk (by Eric Fiselier):
Fix std::pair on FreeBSD Summary: FreeBSD ships an old ABI for std::pair which requires that it have non-trivial copy/move constructors. Currently the non-trivial copy/move is achieved by providing explicit definitions of the constructors. This is problematic because it means the constructors don't SFINAE properly. In order to SFINAE copy/move constructors they have to be explicitly defaulted and hense non-trivial. This patch attempts to provide SFINAE'ing copy/move constructors for std::pair while still making them non-trivial. It does this by adding a base class with a non-trivial copy constructor and then allowing pair's constructors to be generated by the compiler. This also allows the constructors to be constexpr. Reviewers: emaste, theraven, rsmith, dim Subscribers: cfe-commits Differential Revision: https://reviews.llvm.org/D25389
This should fix building www/chromium 57.0.2987.110 on stable/11,
without further hacks. Direct commit to stable/11, since head already
has libc++ 4.0, which includes this fix.
Reported by: cpm