Patchwork [Ada] Freezing actions for object renaming declarations

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Submitter Arnaud Charlet
Date April 11, 2013, 9:47 a.m.
Message ID <20130411094706.GA25810@adacore.com>
Download mbox | patch
Permalink /patch/235658/
State New
Headers show

Comments

Arnaud Charlet - April 11, 2013, 9:47 a.m.
A subtype is created from the renamed object in an object renaming declaration
with an unconstrained nominal subtype.  That subtype must be frozen ahead of
the renaming declaration, to prevent order of elaboration issues when the
nominal subtype of the object has unknown discriminants. The renamed object
itself has already frozen the type.

The following must compile quietly in ada 2012 mode:

with Q;
procedure P is
   subtype Name is String
      with Dynamic_Predicate => (for all C of Name => C not in 'a'..'z');

   package My_Q is new Q (Name);

begin
   null;
end;
---
generic
   type Key_Type (<>) is private;
package Q is

   type Key_Access is access Key_Type;

   type Cursor is record
      Key : Key_Access;
   end record;

   procedure Query_Element (Position : Cursor);
end Q;
---
package body Q is
   procedure Query_Element (Position : Cursor) is
      K : Key_Type renames Position.Key.all;
   begin
      null;
   end;
end Q;

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

2013-04-11  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch8.adb (Check_Constrained_Object): If a subtype is created
	from the renamed object in an object renaming declaration with
	an unconstrained nominal subtype, freeze the created subtype at
	once, to prevent order of elaboration issues in the backend.

Patch

Index: sem_ch8.adb
===================================================================
--- sem_ch8.adb	(revision 197746)
+++ sem_ch8.adb	(working copy)
@@ -792,6 +792,12 @@ 
                      Make_Subtype_From_Expr (Nam, Typ)));
                Rewrite (Subtype_Mark (N), New_Occurrence_Of (Subt, Loc));
                Set_Etype (Nam, Subt);
+
+               --  Freeze subtype at once, to prevent order of elaboration
+               --  issues in the backend. The renamed object exists, so its
+               --  type is already frozen in any case.
+
+               Freeze_Before (N, Subt);
             end if;
          end if;
       end Check_Constrained_Object;