VNCE, chapter 13 (Conformance), page 228

     Let `T' be an expanded type other  than  a  Bit_type.
     Then:


        1 If the base type `BT'  of  `T'  is  a  reference
          type,  `BT'  conforms  directly  to  `T' and `T'
          conforms directly to `BT'.

        2 If `T' is `INTEGER',  it  conforms  directly  to
          `REAL'  and  `DOUBLE',  and  if `T' is `REAL' it
          conforms directly to `DOUBLE'.

          Other than  implied  by  these  cases,  no  type
          conforms  directly  to  `T',  and  `T'  conforms
          directly to no type.