[m-users.] Is this a compiler bug?

Hi.

The following program fails to compile:

:- module pairbug.
:- interface.
:- import_module io.

:- pred main(io::di, io::uo) is det.

:- implementation.
:- import_module pair, string.

:- pred p(pair(string, string)::out, io::di, io::uo) is det.
p("foo" - "bar", !IO).

main(!IO) :-
    p(A - B, !IO).    % <- line 19


The message is:

pairbug.m:006: In `main'(di, uo):
pairbug.m:006:   error: determinism declaration not satisfied.
pairbug.m:006:   Declared `det', inferred `semidet'.
pairbug.m:006:   The reason for the difference is the following.
pairbug.m:019:   In argument 1 of call to predicate `pairbug.p'/3:
pairbug.m:019:   unification with `V_9' can fail.


But the following works:

main(!IO) :-
    p(P, !IO),
    P = A - B.


This must either be something stupid or a compiler bug...


Cheers
Volker

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