VYCQ, section 8.15.8 (ECMA-367, 2nd edition)

     Conversion Query rule

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

        1 fn is the name of a query f of C.

        2 If C is not generic, CT does not conform to TARGET.
        
        3 If C is generic, the type obtained from CT by replacing every
          formal generic parameter by its constraint does not conform to
          TARGET.
        
        4 TARGET's base class is different from the base class of any other
          conversion type listed for a Conversion_query in the Converters
          clause of C.
        
        5 The specification of the base class of TARGET does not list a
          conversion procedurespecifying a type of base class C.
        
        6 f has no formal argument.
        
        7 The result type of f conforms to TARGET.
        
        8 TARGET involves no anchored type.

        9 TARGET is attached.