[over.match.ref]

Under the conditions specified in [dcl.init.ref], a reference can be bound directly to the result of applying a conversion function to an initializer expression.

Overload resolution is used to select the conversion function to be invoked.

Assuming that “reference to cv1 T” is the type of the reference being initialized, the candidate functions are selected as follows:

(1.1)
Let R be a set of types including
- (1.1.1)
  “lvalue reference to cv2 T2” (when initializing an lvalue reference or an rvalue reference to function) and
- (1.1.2)
  “cv2 T2” and “rvalue reference to cv2 T2” (when initializing an rvalue reference or an lvalue reference to function)
for any T2.
The permissible types for non-explicit conversion functions are the members of R where “cv1 T” is reference-compatible ([dcl.init.ref]) with “cv2 T2”.

For direct-initialization, the permissible types for explicit conversion functions are the members of R where T2 can be converted to type T with a (possibly trivial) qualification conversion ([conv.qual]); otherwise there are none.

12 Overloading [over]

12.2 Overload resolution [over.match]

12.2.2 Candidate functions and argument lists [over.match.funcs]

12.2.2.7 Initialization by conversion function for direct reference binding [over.match.ref]