C++ Overload resolution Overloading on Forwarding Reference
Example
You must be very careful when providing a forwarding reference overload as it may match too well:
struct A {
A() = default; // #1
A(A const& ) = default; // #2
template <class T>
A(T&& ); // #3
};
The intent here was that A is copyable, and that we have this other constructor that might initialize some other member. However:
A a; // calls #1
A b(a); // calls #3!
There are two viable matches for the construction call:
A(A const& ); // #2
A(A& ); // #3, with T = A&
Both are Exact Matches, but #3 takes a reference to a less cv-qualified object than #2 does, so it has the better standard conversion sequence and is the best viable function.
The solution here is to always constrain these constructors (e.g. using SFINAE):
template <class T,
class = std::enable_if_t<!std::is_convertible<std::decay_t<T>*, A*>::value>
>
A(T&& );
The type trait here is to exclude any A or class publicly and unambiguously derived from A from consideration, which would make this constructor ill-formed in the example described earlier (and hence removed from the overload set). As a result, the copy constructor is invoked - which is what we wanted.
