[Ada] Add missing run-time check on class-wide assignment

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

In case of the assignment to a class-wide object, before the assignment we
generate a run-time check to ensure that the tag of the Target is covered
by the tag of the source. For example, considering the following program:
--
procedure Do_Test is
   package Pkg is
     type Root is abstract tagged null record;
     procedure Proc (A: in Root; Output: out Root'Class) is abstract;
     type B_T is new Root with record
       B_Data : String (1 .. 1024);
     end record;
     procedure Proc (B: in B_T; Output: out Root'Class);
     type C_T is new Root with record
       C_Float : Float := 0.0;
     end record;
     procedure Proc (C: in C_T; Output: out Root'Class);
   end Pkg;
   package body Pkg is
     procedure Proc (B: in B_T; Output: out Root'Class) is
        B_Result : B_T;
     begin
        Output := Root'Class (B_Result);    -- 1 --
     end Proc;
     procedure Proc (C: in C_T; Output: out Root'Class) is
     begin
        null;
     end;
   end Pkg;
   use Pkg;
   B : B_T;
   C : C_T;
begin
  Proc (B, C);
end Do_Test;
--
Its execution generates the following error at -1-:
raised CONSTRAINT_ERROR : do_test.adb:18 tag check failed

2005-12-05  Javier Miranda  <miranda@adacore.com>

	* exp_ch5.adb (Expand_N_Assignment_Statement): In case of tagged types
	and the assignment to a class-wide object, before the assignment we
	generate a run-time check to ensure that the tag of the Target is
	covered by the tag of the source.

Attachment: difs.12
Description: Text document