% $ mmc -e missing_constraints.m
% missing_constraints.m:030: In clause for type class method implementation:
% missing_constraints.m:030:   unsatisfiable typeclass constraints:
% missing_constraints.m:030:   `missing_constraints.t1(int)' and
% missing_constraints.m:030:   `missing_constraints.t2(int)'.
%
:- module missing_constraints.

:- interface.

:- typeclass t1(T) where [].
:- typeclass t2(T) where [].

:- typeclass t3(T) where [
    pred q(T::in, T::in) is semidet
].

:- pred p(T::in, T::in) is semidet <= (t1(T), t2(T)).

:- implementation.

p(A, A).

:- instance t1(int) where [].
% Uncomment below for this to compile
%:- instance t2(int) where [].

:- instance t3(int) where [
    (q(A0, A) :-
        p(A0, A)
    )
].