[Ada] Reject boxes in delta record aggregates

[Pierre-Marie de Rodat via Gcc-patches]

Implement Ada 2022 4.3.1(17.3/5), prevents box compound delimiter <> to
appear in record delta aggregates.

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

gcc/ada/

        * sem_aggr.adb (Resolve_Delta_Record_Aggregate): Reject boxes in
        record delta aggregates.
diff --git a/gcc/ada/sem_aggr.adb b/gcc/ada/sem_aggr.adb
--- a/gcc/ada/sem_aggr.adb
+++ b/gcc/ada/sem_aggr.adb
@@ -3545,7 +3545,19 @@ package body Sem_Aggr is
          end loop;
 
          pragma Assert (Present (Comp_Type));
-         Analyze_And_Resolve (Expression (Assoc), Comp_Type);
+
+         --  A record_component_association in record_delta_aggregate shall not
+         --  use the box compound delimiter <> rather than an expression; see
+         --  RM 4.3.1(17.3/5).
+
+         pragma Assert (Present (Expression (Assoc)) xor Box_Present (Assoc));
+
+         if Box_Present (Assoc) then
+            Error_Msg_N
+              ("'<'> in record delta aggregate is not allowed", Assoc);
+         else
+            Analyze_And_Resolve (Expression (Assoc), Comp_Type);
+         end if;
          Next (Assoc);
       end loop;
    end Resolve_Delta_Record_Aggregate;