[Ada] Improper visibility of loop parameter with Iterable aspect

[Pierre-Marie de Rodat <derodat@adacore.com>]

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This patch fixes a visibility bug in the expansion of a loop over an object
whose type carries the GNAT-specific Iterable aspect. Prior to this patch,
the loop variable remained visible after exit from the loop.

Compiling iterator_test.adb must yield:

   iterator_test.adb:26:61: "number" is not visible
   iterator_test.adb:26:61: non-visible declaration at line 21
   iterator_test.adb:26:61: non-visible declaration at line 16

---
with Ada.Text_IO;
with iterators; use iterators;
procedure iterator_test is

   it_test : constant my_iterator := create_it(from => 3, to => 10);

begin

   -- reference loop
   for number in 1 .. 10 loop
      Ada.Text_IO.Put_Line (Integer'Image (number));
   end loop;

   -- gas day iterations
   Ada.Text_IO.Put_Line ("Test foreward iteration");
   for number in it_test loop
      Ada.Text_IO.Put_Line (Integer'Image (number));
   end loop;

   Ada.Text_IO.Put_Line ("Test backward iteration");
   for number in create_it(from => 10, to => 3)
   loop
      Ada.Text_IO.Put_Line (Integer'Image (number));
   end loop;

   Ada.Text_IO.Put_Line ("Out of scope?! " & Integer'Image (number));

end iterator_test;
---
package iterators is

   type my_iterator is private with Iterable =>
        (first       => iterator_first,
         next        => iterator_next,
         has_element => iterator_has_element);

   function create_it (from : Integer; to : Integer) return my_iterator;

   function iterator_first (it : my_iterator) return Integer;

   function iterator_next (it : my_iterator; prev_item : Integer)
      return Integer;

   function iterator_has_element (it : my_iterator; prev_item : Integer) 
     return Boolean;

private
   type my_iterator is record
      from : Integer;
      to   : Integer;
   end record;

end iterators;

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

2017-12-05  Ed Schonberg  <schonberg@adacore.com>

	* exp_ch5.adb (Expand_Formal_Container_Loop): Ensure that the loop
	parameter becomes invisible after analyzing the loop, which has been
	rewritten as a while-loop.


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Index: exp_ch5.adb
===================================================================
--- exp_ch5.adb	(revision 255408)
+++ exp_ch5.adb	(working copy)
@@ -3135,6 +3135,7 @@
 
       Advance   : Node_Id;
       Init_Decl : Node_Id;
+      Init_Name : Entity_Id;
       New_Loop  : Node_Id;
 
    begin
@@ -3169,7 +3170,8 @@
       --  the loop.
 
       Analyze (Init_Decl);
-      Set_Ekind (Defining_Identifier (Init_Decl), E_Loop_Parameter);
+      Init_Name := Defining_Identifier (Init_Decl);
+      Set_Ekind (Init_Name, E_Loop_Parameter);
 
       --  The cursor was marked as a loop parameter to prevent user assignments
       --  to it, however this renders the advancement step illegal as it is not
@@ -3182,9 +3184,11 @@
       --  Because we have to analyze the initial declaration of the loop
       --  parameter multiple times its scope is incorrectly set at this point
       --  to the one surrounding the block statement - so set the scope
-      --  manually to be the actual block statement.
+      --  manually to be the actual block statement, and indicate that it is
+      --  not visible after the block has been analyzed.
 
-      Set_Scope (Defining_Identifier (Init_Decl), Entity (Identifier (N)));
+      Set_Scope (Init_Name, Entity (Identifier (N)));
+      Set_Is_Immediately_Visible (Init_Name, False);
    end Expand_Formal_Container_Loop;
 
    ------------------------------------------