Error with Opaque types in GNU Modula-2?

[T . Kurt Bond]

I've got the following program consisting of three files. First,
MinOpaque.def:

DEFINITION MODULE MinOpaque;
TYPE Opaque;
PROCEDURE Make (VAR o: Opaque; i: INTEGER);
END MinOpaque.

Next, MinOpaque.mod:

IMPLEMENTATION MODULE MinOpaque;
FROM Storage IMPORT ALLOCATE, DEALLOCATE;
FROM SYSTEM IMPORT TSIZE;

TYPE Opaque = POINTER TO INTEGER;

PROCEDURE Make (VAR o: Opaque; i: INTEGER);
BEGIN
     ALLOCATE (o, TSIZE (Opaque));
     o^ := i;
END Make;
          
END MinOpaque.

Finally, TryMinOpaque.mod:

MODULE TryMinOpaque;
FROM InOut IMPORT WriteString, WriteLn, WriteInt;
FROM MinOpaque IMPORT Opaque, Make;

VAR o: Opaque;

BEGIN
     Make (o, 10);
     o^ := 20;                  (* I thought this was illegal. *)
     WriteString ("o^: "); WriteInt (o^, 0); WriteLn;
END TryMinOpaque.

I thought you didn't have access to the internals of type Opaque in
Modula-2.  Is this a bug in gm2, or am I misunderstanding?

$ gm2 --version
gm2 (GCC) 14.2.1 20250110 (Red Hat 14.2.1-7)
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

This is on Fedora 41 with 
$ uname -a
Linux atla 6.12.8-200.fc41.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Jan  2 19:26:03 
UTC 2025 x86_64 GNU/Linux

-- 
T. Kurt Bond, tkurtbond@gmail.com, tkurtbond.github.io, consp.org, & tkb.tx0.org