[Ada] Fix ICE on subprogram defined with null exclusion

[Eric Botcazou <ebotcazou@adacore.com>]

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It's a regression present on the mainline and 7 branch: the compiler aborts on 
a package containing a declaration of a subprogram taking an access parameter 
with null exclusion and a reference to the Access attribute applied to this 
subprogram.

Tested on x86_64-suse-linux, applied on the mainline and 7 branch.


2017-05-22  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/decl.c (gnat_to_gnu_entity): Skip regular processing
	for Itypes that are E_Access_Subtype.
	<E_Access_Subtype>: Use the DECL of the base type directly.


2017-05-22  Pierre-Marie de Rodat  <derodat@adacore.com>

	* gnat.dg/specs/not_null1.ads: New test.

-- 
Eric Botcazou

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-- { dg-do compile }

package Not_Null1 is
   type T is null record;
   type T_Access is access all T;

   procedure Proc (This : in not null T_Access) is null;

   type Proc_Access is access procedure (This : in not null T_Access);
   PA : Proc_Access := Proc'Access;
end Not_Null1;

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Index: gcc-interface/decl.c
===================================================================
--- gcc-interface/decl.c	(revision 248321)
+++ gcc-interface/decl.c	(working copy)
@@ -310,11 +310,13 @@ gnat_to_gnu_entity (Entity_Id gnat_entit
   /* Contains the list of attributes directly attached to the entity.  */
   struct attrib *attr_list = NULL;
 
-  /* Since a use of an Itype is a definition, process it as such if it
-     is not in a with'ed unit.  */
+  /* Since a use of an Itype is a definition, process it as such if it is in
+     the main unit, except for E_Access_Subtype because it's actually a use
+     of its base type, see below.  */
   if (!definition
       && is_type
       && Is_Itype (gnat_entity)
+      && Ekind (gnat_entity) != E_Access_Subtype
       && !present_gnu_tree (gnat_entity)
       && In_Extended_Main_Code_Unit (gnat_entity))
     {
@@ -3823,7 +3825,8 @@ gnat_to_gnu_entity (Entity_Id gnat_entit
     case E_Access_Subtype:
       /* We treat this as identical to its base type; any constraint is
 	 meaningful only to the front-end.  */
-      gnu_type = gnat_to_gnu_type (Etype (gnat_entity));
+      gnu_decl = gnat_to_gnu_entity (Etype (gnat_entity), NULL_TREE, false);
+      saved = true;
 
       /* The designated subtype must be elaborated as well, if it does
 	 not have its own freeze node.  But designated subtypes created
@@ -3855,8 +3858,6 @@ gnat_to_gnu_entity (Entity_Id gnat_entit
 	    gnat_to_gnu_entity (Directly_Designated_Type (gnat_entity),
 				NULL_TREE, false);
 	}
-
-      maybe_present = true;
       break;
 
     /* Subprogram Entities