[Ada] Constant-folding and discriminants

[Arnaud Charlet <charlet at adacore dot com>]

The compiler tries to optimize comparisons involving expressions that are not
compile-time known, but can be recognized as identical, for example references
to the same variable. If the expressions are references to the same discriminant
the expressions cannot be assumed to be equal, because they may denote the
discriminant of two different instances of the same discriminated type, used
for example as a component bound.

The following must compile and execute quietly:

procedure No_Fold is
   type Arr is array (integer range <>) of Integer;
   type Rec (Discr : Positive) is record
      Field : Arr (1 .. Discr);
   end record;

   R1 : Rec (3) := (3, (others => 0));
   R2 : Rec (7) := (7, (others => 0));

   procedure Compare (X, Y : Rec) is
   begin
      if X.Discr < Y.Discr
         and then not (X.Field'Last < Y.Field'Last)
      then
         raise Program_Error;
      end if;
   end Compare;
begin
   Compare (R1, R2);
end;

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

2010-09-09  Ed Schonberg  <schonberg@adacore.com>

	* sem_eval.adb (Is_Same_Value): Two occurrences of the same
	discriminant cannot be assumed to be the same value because they may
	refer to bounds of a component of two different instances of a
	discriminated type.

Attachment: difs
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