[Ada] Spurious error with null Abstract_State

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch corrects the mechanism which ensures that a package with a null
Abstract_State does not introduce hidden state, by ignoring internal states
and variables because they do not represent the "source" hidden state.

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

2018-07-16  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_util.adb (Check_No_Hidden_State): Ignore internally-generated
	states and variables.

gcc/testsuite/

	* gnat.dg/abstract_state1.adb, gnat.dg/abstract_state1.ads: New
	testcase.

--- gcc/ada/sem_util.adb
+++ gcc/ada/sem_util.adb
@@ -3228,6 +3228,13 @@ package body Sem_Util is
    begin
       pragma Assert (Ekind_In (Id, E_Abstract_State, E_Variable));
 
+      --  Nothing to do for internally-generated abstract states and variables
+      --  because they do not represent the hidden state of the source unit.
+
+      if not Comes_From_Source (Id) then
+         return;
+      end if;
+
       --  Find the proper context where the object or state appears
 
       Scop := Scope (Id);

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/abstract_state1.adb
@@ -0,0 +1,5 @@
+--  { dg-do compile }
+
+package body Abstract_State1 is
+   procedure Foo is null;
+end Abstract_State1;

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/abstract_state1.ads
@@ -0,0 +1,24 @@
+package Abstract_State1
+  with Abstract_State => null,
+       Initializes    => null
+is
+   type Complex (B : Boolean) is tagged private;
+   type No_F is tagged private;
+   X : constant No_F;
+
+   procedure Foo;
+
+private
+   type Complex (B : Boolean) is tagged record
+      G : Integer;
+      case B is
+         when True =>
+            F : Integer;
+         when False =>
+            null;
+      end case;
+   end record;
+
+   type No_F is new Complex (False) with null record;
+   X : constant No_F := (B => False, G => 7);
+end Abstract_State1;