[Ada] Crash on inherited component in type extension in generic unit.

[Pierre-Marie de Rodat]

Compiler aborts on an instance body that has a reference to a selected
component of a local type extension, when the component is inherited
from an ancestor type declared outside of the generic being
instantiated.

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

gcc/ada/

        * exp_ch3.adb (Expand_Record_Extension): Set Parent_Subtype on
        the type extension when within a generic unit, even though
        expansion is disabled, to allow for proper resolution of
        selected components inherited from an ancestor.
diff --git a/gcc/ada/exp_ch3.adb b/gcc/ada/exp_ch3.adb
--- a/gcc/ada/exp_ch3.adb
+++ b/gcc/ada/exp_ch3.adb
@@ -7782,9 +7782,14 @@ package body Exp_Ch3 is
       --  Expand_Record_Extension is called directly from the semantics, so
       --  we must check to see whether expansion is active before proceeding,
       --  because this affects the visibility of selected components in bodies
-      --  of instances.
+      --  of instances. Within a generic we still need to set Parent_Subtype
+      --  link because the visibility of inherited components will have to be
+      --  verified in subsequent instances.
 
       if not Expander_Active then
+         if Inside_A_Generic and then Ekind (T) = E_Record_Type then
+            Set_Parent_Subtype (T, Etype (T));
+         end if;
          return;
       end if;