I've been playing with trying to create a safe-pointer type for SPARK, where
SPARK doesn't know that access types are involved. So I've tried

private with Ada.Finalization;

generic -- SRC.Pointers
type Object (<>) is private;
package SRC.Pointers with SPARK_Mode
is
type Pointer is tagged private with
Default_Initial_Condition => Pointer.Is_Null;

function Is_Null ...

...

private -- SRC.Pointers
pragma SPARK_Mode (Off);

type Safe_Group;

type Name is access Safe_Group;

type Pointer is new Ada.Finalization.Controlled with record
Ptr : Name;
end record;

overriding procedure Adjust (Item : in out Pointer);
overriding procedure Finalize (Item : in out Pointer);
end SRC.Pointers;

My understanding is that SPARK will not look at the private part or the body.
All proofs will assume that the operations are correct.

When I instantiate it in a SPARK unit:

package Pointers is new SRC.Pointers (Object => Fixed);

and run

gnatprove --level=4 -Psrc

I get errors such as

value Off was set for SPARK_Mode on "Adjust" at src-pointers.ads:24

implying that SPARK is looking at the private part of SRC.Pointers. I can work
around that by putting the instantiation in an internal pkg with SPARK_Mode => Off:

package SPARK_Control with SPARK_Mode => Off
is
package Pointers is new SRC.Pointers (Object => Fixed);
end SPARK_Control;

package Pointers renames SPARK_Control.Pointers;

but then I get errors such as

"Pointer" is not allowed in SPARK

"Object" is not allowed in SPARK

which don't make much sense. How can I get SPARK to ignore the private part and
body of SRC.Pointers?

--
Jeff Carter
"You cheesy lot of second-hand electric donkey-bottom biters."
Monty Python & the Holy Grail
14