Comment by detrino

Returning a stack allocated value is a move in C++11/14. It is also true that in many circumstances a compiler may optimize that move away using RVO/NRVO.

Consider this example: http://ideone.com/T872Oj

In F1 the compiler can NRVO away the move.

In F2 it cannot.

Both will fail to compile if you delete the move constructor.