Otherwise, C++11-compliant compilers may select PosixErrorOr(const T&)
as the relevant constructor, and fail because std::vector<Mapping> is
not copyable (because Mapping is not copyable).
This is a C++11 defect that is fixed in C++14 (and in C++11 mode for
Clang, but not GCC). See DR 1579,
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579.
PiperOrigin-RevId: 230767401
Change-Id: I65f481f5188d91db6cbbbd65ed0a60bc55df3401
Jamie Liu
Ian Gudger
Brian Geffon