The compiler reports an spurious error processing a derived type
of a non-tagged record type that has discriminants, pragma
Unchecked_Union and pragma Convention C.

After this patch the following test compiles silently.

procedure Conversion is
   type small_array is array (0 .. 2) of Integer;
   type big_array   is array (0 .. 3) of Integer;

   type small_record is record
      field1 : aliased Integer     := 0;
      field2 : aliased small_array := (0, 0, 0);
   end record;

   type big_record is record
      field1 : aliased Integer   := 0;
      field2 : aliased big_array := (0, 0, 0, 0);
   end record;

   type myUnion (discr : Integer := 0) is record
      case discr is
         when 0 =>
            record1 : aliased small_record;
         when others =>
            record2 : aliased big_record;
      end case;
   end record;

   type UU_myUnion3 (discr : Integer := 0) is new myUnion (discr); -- Test
   pragma Unchecked_Union (UU_myUnion3);
   pragma Convention (C, UU_myUnion3);

   procedure Convert (A : in UU_myUnion3; B : out UU_myUnion3);
   pragma Import (C, Convert);

begin
   null;
end Conversion;

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

2017-09-29  Javier Miranda  <miranda@adacore.com>

	* sem_ch3.adb (Replace_Components): Update references to discriminants
	located in variant parts inherited from the parent type.

gcc/testsuite/

2017-09-29  Javier Miranda  <miranda@adacore.com>

	* gnat.dg/unchecked_union2.adb: New testcase.