DR1579 says that when returning a id-expression from a function, we should attempt to return said expression by move first, then fallback to copy.
This patch does this by generalizing PerformMoveOrCopyElision, which previously did this when returning a id-expression that was a parameter.
This patch allows the following code to compile, for example:
struct Base {}; struct Derived : public Base {}; std::unique_ptr<Base> f() { std::unique_ptr<Derived> d; return d; }
The DR in question: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579
Pointed out by PR27785.
Thanks!
Constructable -> Constructible.