[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 the set of all
- (1.1.1)
  lvalue reference types (when converting to an lvalue) and
- (1.1.2)
  non-reference types and rvalue reference types (when converting to an rvalue or an lvalue of function type).
The permissible types for non-explicit conversion functions are the members of R having the form “cv T2” or “reference to cv2 T2” 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 having the form “cv T2” or “reference to cv2 T2” 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]