[Ada] Fix spurious error on implicit dereference for private type

[Pierre-Marie de Rodat]

This is a fallout of the earlier work on the handling of Ada 95
implicit dereferences.  The dereferences are now made explicit
only at the final stage of type resolution and this slightly
changes the way they are analyzed during semantic analysis.

In particular, in the case of an implicit dereference of a component
inherited in a tagged extension from a private parent and used as
actual type in an instance, Analyze_Selected_Component has specific
code to recognize this situation after the interpretation as prefixed
call has failed.

This tentative interpretation is done by Try_Object_Operation and
it turns out that the procedure may be partially destructive for the
parent of the analyzed node in the case where it is a subprogram call
or an indexed component.

This change makes sure that the partial damage is undone on failure.

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

2020-06-16  Eric Botcazou  <ebotca...@adacore.com>

gcc/ada/

        * sem_ch4.adb (Transform_Object_Operation): Document that it
        may be partially destructive for the parent of the node.
        (Try_Object_Operation): Undo the changes made above on failure.
--- gcc/ada/sem_ch4.adb
+++ gcc/ada/sem_ch4.adb
@@ -8460,7 +8460,9 @@ package body Sem_Ch4 is
       --  Transform Obj.Operation (X, Y, ...) into Operation (Obj, X, Y ...).
       --  Call_Node is the resulting subprogram call, Node_To_Replace is
       --  either N or the parent of N, and Subprog is a reference to the
-      --  subprogram we are trying to match.
+      --  subprogram we are trying to match. Note that the transformation
+      --  may be partially destructive for the parent of N, so it needs to
+      --  be undone in the case where Try_Object_Operation returns false.
 
       function Try_Class_Wide_Operation
         (Call_Node       : Node_Id;
@@ -8731,7 +8733,7 @@ package body Sem_Ch4 is
             --  example:
             --            Some_Subprogram (..., Obj.Operation, ...)
 
-            and then Name (Parent_Node) = N
+            and then N = Name (Parent_Node)
          then
             Node_To_Replace := Parent_Node;
 
@@ -9769,8 +9771,20 @@ package body Sem_Ch4 is
          return True;
 
       else
-         --  There was no candidate operation, so report it as an error
-         --  in the caller: Analyze_Selected_Component.
+         --  There was no candidate operation, but Analyze_Selected_Component
+         --  may continue the analysis so we need to undo the change possibly
+         --  made to the Parent of N earlier by Transform_Object_Operation.
+
+         declare
+            Parent_Node : constant Node_Id := Parent (N);
+
+         begin
+            if Node_To_Replace = Parent_Node then
+               Remove (First (Parameter_Associations (New_Call_Node)));
+               Set_Parent
+                 (Parameter_Associations (New_Call_Node), Parent_Node);
+            end if;
+         end;
 
          return False;
       end if;