Determines if T has an optimized path T::clone_from(const T&) that can be used from clone_into.

This is largely an implementation detail of clone_into as it can be called on all types that satisfy Clone. But it can be useful for verifying in a static assert that a type is written correctly and the optimization will be used.

Evaluates to true if the type is Copy, as the copy assignment operator acts like clone_from method. It return false if the type is Copy and also has a clone_from method, as it becomes ambiguous and is thus invalid.