[Ada] Spurious error in instantiation with aggregate and private ancestor

[Pierre-Marie de Rodat]

Compiler rejects an instantiation but accepts the corresponding generic
unit, when it includes a declaration for a private type whose full view
is a record type with a controlled component, and the full view of an
object of the type is given by an aggregate with default-initialized
components.

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

gcc/ada/

        * sem_aggr.adb (Resolve_Record_Aggregate, Step_5): Do not check
        for the need to use an extension aggregate for a given component
        when within an instance and the type of the component hss a
        private ancestor: the instantiation is legal if the generic
        compiles, and spurious errors may be generated otherwise.
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
@@ -5028,12 +5028,19 @@ package body Sem_Aggr is
                Prepend_Elmt (Parent_Typ, To => Parent_Typ_List);
                Parent_Typ := Etype (Parent_Typ);
 
+               --  Check whether a private parent requires the use of
+               --  an extension aggregate. This test does not apply in
+               --  an instantiation: if the generic unit is legal so is
+               --  the instance.
+
                if Nkind (Parent (Base_Type (Parent_Typ))) =
                                         N_Private_Type_Declaration
                  or else Nkind (Parent (Base_Type (Parent_Typ))) =
                                         N_Private_Extension_Declaration
                then
-                  if Nkind (N) /= N_Extension_Aggregate then
+                  if Nkind (N) /= N_Extension_Aggregate
+                    and then not In_Instance
+                  then
                      Error_Msg_NE
                        ("type of aggregate has private ancestor&!",
                         N, Parent_Typ);