The following does not compile:
package Test is
type X is new Positive;
type X_Access is not null access constant X;
Y : constant X_Access;
private
Y_Val : aliased constant X := 1;
Y : constant X_Access := Y_Val'Access;
end Test;
with the following diagnostics:
test.ads:6:09: warning: (Ada 2005) null-excluding objects must be initialized
test.ads:6:09: warning: "Constraint_Error" will be raised at run time
test.ads:11:09: "Y" conflicts with declaration at line 6
If "not null" modifier is removed, everything is fine.
As I can see from the following code, the semantic analyzer sees a deferred
constant and (erroneously) assumes that it's an uninitialized object. It then
inserts an implicit null initialization. And when it comes to a full definition
of the constant, the compiler thinks it has been already defined:
(gcc/ada/checks.adb, procedure Null_Exclusion_Static_Checks, line 2728):
if K = N_Object_Declaration
and then No (Expression (N))
and then not No_Initialization (N)
then
-- Add an expression that assigns null. This node is needed by
-- Apply_Compile_Time_Constraint_Error, which will replace this with
-- a Constraint_Error node.
Set_Expression (N, Make_Null (Sloc (N)));
Set_Etype (Expression (N), Etype (Defining_Identifier (N)));
Apply_Compile_Time_Constraint_Error
(N => Expression (N),
Msg => "(Ada 2005) null-excluding objects must be initialized?",
Reason => CE_Null_Not_Allowed);
end if;
The code can be traced back to at least gcc 4.1.2
--
Summary: deferred constants of not null access types are not
possible
Product: gcc
Version: 4.3.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: ada
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: Artem dot Andreev at oktetlabs dot ru
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38975
