When a parameter of reference type binds directly ([dcl.init.ref]) to an argument expression, the implicit conversion sequence is the identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion sequence is a derived-to-base Conversion ([over.best.ics]). [ Example:
struct A {};
struct B : public A {} b;
int f(A&);
int f(B&);
int i = f(b); // calls f(B&), an exact match, rather than
// f(A&), a conversion
— end example ] If the parameter binds directly to the result of applying a conversion function to the argument expression, the implicit conversion sequence is a user-defined conversion sequence ([over.ics.user]), with the second standard conversion sequence either an identity conversion or, if the conversion function returns an entity of a type that is a derived class of the parameter type, a derived-to-base Conversion.
When a parameter of reference type is not bound directly to an argument expression, the conversion sequence is the one required to convert the argument expression to the referenced type according to [over.best.ics]. Conceptually, this conversion sequence corresponds to copy-initializing a temporary of the referenced type with the argument expression. Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion.
Except for an implicit object parameter, for which see [over.match.funcs], a standard conversion sequence cannot be formed if it requires binding an lvalue reference other than a reference to a non-volatile const type to an rvalue or binding an rvalue reference to an lvalue other than a function lvalue. [ Note: This means, for example, that a candidate function cannot be a viable function if it has a non-const lvalue reference parameter (other than the implicit object parameter) and the corresponding argument would require a temporary to be created to initialize the lvalue reference (see [dcl.init.ref]). — end note ]
Other restrictions on binding a reference to a particular argument that are not based on the types of the reference and the argument do not affect the formation of a standard conversion sequence, however. [ Example: a function with an “lvalue reference to int” parameter can be a viable candidate even if the corresponding argument is an int bit-field. The formation of implicit conversion sequences treats the int bit-field as an int lvalue and finds an exact match with the parameter. If the function is selected by overload resolution, the call will nonetheless be ill-formed because of the prohibition on binding a non-const lvalue reference to a bit-field ([dcl.init.ref]). — end example ]