[COMMITTED] ada: Fix small inaccuracy in implementation of B.3.3(20/2)

["Marc Poulhiès" <poulhies@adacore.com>]

From: Eric Botcazou <ebotcazou@adacore.com>

This is the clause about inferable discriminants in unchecked unions.

gcc/ada/

	* sem_util.adb (Has_Inferable_Discriminants): In the case of a
	component with a per-object constraint, also return true if the
	enclosing object is not of an unchecked union type.
	In the default case, remove a useless call to Base_Type.

Tested on x86_64-pc-linux-gnu, committed on master.

---
 gcc/ada/sem_util.adb | 35 ++++++++++++++---------------------
 1 file changed, 14 insertions(+), 21 deletions(-)

diff --git a/gcc/ada/sem_util.adb b/gcc/ada/sem_util.adb
index d9ea00e53cb..736751f5fae 100644
--- a/gcc/ada/sem_util.adb
+++ b/gcc/ada/sem_util.adb
@@ -12272,33 +12272,26 @@ package body Sem_Util is
    begin
       --  For selected components, the subtype of the selector must be a
       --  constrained Unchecked_Union. If the component is subject to a
-      --  per-object constraint, then the enclosing object must have inferable
-      --  discriminants.
+      --  per-object constraint, then the enclosing object must either be
+      --  a regular discriminated type or must have inferable discriminants.
 
       if Nkind (N) = N_Selected_Component then
-         if Has_Per_Object_Constraint (Entity (Selector_Name (N))) then
-
-            --  A small hack. If we have a per-object constrained selected
-            --  component of a formal parameter, return True since we do not
-            --  know the actual parameter association yet.
-
-            if Prefix_Is_Formal_Parameter (N) then
-               return True;
-
-            --  Otherwise, check the enclosing object and the selector
-
-            else
-               return Has_Inferable_Discriminants (Prefix (N))
-                 and then Has_Inferable_Discriminants (Selector_Name (N));
-            end if;
-
          --  The call to Has_Inferable_Discriminants will determine whether
          --  the selector has a constrained Unchecked_Union nominal type.
 
-         else
-            return Has_Inferable_Discriminants (Selector_Name (N));
+         if not Has_Inferable_Discriminants (Selector_Name (N)) then
+            return False;
          end if;
 
+         --  A small hack. If we have a per-object constrained selected
+         --  component of a formal parameter, return True since we do not
+         --  know the actual parameter association yet.
+
+         return not Has_Per_Object_Constraint (Entity (Selector_Name (N)))
+           or else not Is_Unchecked_Union (Etype (Prefix (N)))
+           or else Has_Inferable_Discriminants (Prefix (N))
+           or else Prefix_Is_Formal_Parameter (N);
+
       --  A qualified expression has inferable discriminants if its subtype
       --  mark is a constrained Unchecked_Union subtype.
 
@@ -12310,7 +12303,7 @@ package body Sem_Util is
       --  Unchecked_Union nominal subtype.
 
       else
-         return Is_Unchecked_Union (Base_Type (Etype (N)))
+         return Is_Unchecked_Union (Etype (N))
            and then Is_Constrained (Etype (N));
       end if;
    end Has_Inferable_Discriminants;
-- 
2.40.0