VYCP, section 8.15.7 (ECMA-367, 2nd edition)

     Conversion Procedure rule

     A Conversion_procedure listing a Feature_name fn and appearing in a
     class C with current typeCT is valid if and only if it satisfies the
     following conditions, applicable to every type SOURCE listed in its
     Type_list:

        1 fn is the name of a creation procedure cp of C.

        2 If C is not generic, SOURCE does not conform to CT.

        3 If C is generic, SOURCE does not conform to the type obtained
          from CT by replacing every formal generic parameter by its
          constraint.

        4 SOURCE's base class is different from the base class of any other
          conversion type listed for a Conversion_procedure in the
          Converters clause of C.

        5 The specification of the base class of SOURCE does not list a
          conversion queryspecifying a type of base class C.

        6 cp has exactly one formal argument, of a type ARG.

        7 SOURCE conforms to ARG.

        8 SOURCE involves no anchored type.