Summary: The static cast and forward were backwards. Previously it happily converted an rvalue to a const lvalue. Now it causes the exact same static_assert failure as std::forward in every rvalue->lvalue case.

Reviewed By: yfeldblum

Differential Revision: D26680263

fbshipit-source-id: f1ca4c4bfdd6333d24fe3c406d5373a2f72dea31