VNCP, chapter 14 (Conformance and convertibility)

    Conversion procedure rule

    It is valid for a creation procedure cp of a class C,
    with current type CT, to be a conversion procedure if
    and only if it satisfies the following four conditions
    for every type SOURCE listed in its Conversion_types:
 
        1 If CT is not generic, SOURCE does not conform
          to CT.

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

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

        4 SOURCE conforms or converts to ARG.

        5 SOURCE's base type is different from the base
          type of any other conversion type listed in
          the Creation_clause of C.

        6 SOURCE is a valid type for conversion