[Ada] Warn when interfaces swapped between full and partial view

[Ada] Warn when interfaces swapped between full and partial view

[Pierre-Marie de Rodat]

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The following package declaration is legal but the declaration of D
leads to performing a tree transformation.  Defining D as `type D is new
B and A with null record` would be consistent with the partial view and
thus does not require any transformation.

This is helpful in the case of generic packages where we fail to
correctly transform the tree.

package E is
   type A is interface;
   type B is interface and A;
   type D is new B with private;
private
   type D is new A and B with null record;
end;

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

gcc/ada/

	* sem_ch3.adb (Derived_Type_Declaration): Introduce a subprogram
	for tree transformation. If a tree transformation is performed,
	then warn that it would be better to reorder the interfaces.

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diff --git a/gcc/ada/sem_ch3.adb b/gcc/ada/sem_ch3.adb
--- a/gcc/ada/sem_ch3.adb
+++ b/gcc/ada/sem_ch3.adb
@@ -17258,10 +17258,46 @@ package body Sem_Ch3 is
         and then Is_Interface (Parent_Type)
       then
          declare
-            Iface               : Node_Id;
             Partial_View        : Entity_Id;
             Partial_View_Parent : Entity_Id;
-            New_Iface           : Node_Id;
+
+            function Reorder_Interfaces return Boolean;
+            --  Look for an interface in the full view's interface list that
+            --  matches the parent type of the partial view, and when found,
+            --  rewrite the full view's parent with the partial view's parent,
+            --  append the full view's original parent to the interface list,
+            --  recursively call Derived_Type_Definition on the full type, and
+            --  return True. If a match is not found, return False.
+            --  ??? This seems broken in the case of generic packages.
+
+            ------------------------
+            -- Reorder_Interfaces --
+            ------------------------
+
+            function Reorder_Interfaces return Boolean is
+               Iface     : Node_Id;
+               New_Iface : Node_Id;
+            begin
+               Iface := First (Interface_List (Def));
+               while Present (Iface) loop
+                  if Etype (Iface) = Etype (Partial_View) then
+                     Rewrite (Subtype_Indication (Def),
+                       New_Copy (Subtype_Indication (Parent (Partial_View))));
+
+                     New_Iface :=
+                       Make_Identifier (Sloc (N), Chars (Parent_Type));
+                     Append (New_Iface, Interface_List (Def));
+
+                     --  Analyze the transformed code
+
+                     Derived_Type_Declaration (T, N, Is_Completion);
+                     return True;
+                  end if;
+
+                  Next (Iface);
+               end loop;
+               return False;
+            end Reorder_Interfaces;
 
          begin
             --  Look for the associated private type declaration
@@ -17282,30 +17318,26 @@ package body Sem_Ch3 is
                then
                   null;
 
-               --  Traverse the list of interfaces of the full-view to look
-               --  for the parent of the partial-view and perform the tree
-               --  transformation.
+               --  Traverse the list of interfaces of the full view to look
+               --  for the parent of the partial view and reorder the
+               --  interfaces to match the order in the partial view,
+               --  if needed.
 
                else
-                  Iface := First (Interface_List (Def));
-                  while Present (Iface) loop
-                     if Etype (Iface) = Etype (Partial_View) then
-                        Rewrite (Subtype_Indication (Def),
-                          New_Copy (Subtype_Indication
-                                     (Parent (Partial_View))));
-
-                        New_Iface :=
-                          Make_Identifier (Sloc (N), Chars (Parent_Type));
-                        Append (New_Iface, Interface_List (Def));
 
-                        --  Analyze the transformed code
+                  if Reorder_Interfaces then
+                     --  Having the interfaces listed in any order is legal.
+                     --  However, the compiler does not properly handle
+                     --  different orders between partial and full views in
+                     --  generic units. We give a warning about the order
+                     --  mismatch, so the user can work around this problem.
 
-                        Derived_Type_Declaration (T, N, Is_Completion);
-                        return;
-                     end if;
+                     Error_Msg_N ("??full declaration does not respect " &
+                                  "partial declaration order", T);
+                     Error_Msg_N ("\??consider reordering", T);
 
-                     Next (Iface);
-                  end loop;
+                     return;
+                  end if;
                end if;
             end if;
          end;