[m-rev.] for review: Fix aborts when checking coercions.

[Peter Wang]

On Thu, 10 Jul 2025 09:46:12 +0200 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> 
> 
> On Thu, 10 Jul 2025 15:54:37 +1000, Peter Wang <novalazy at gmail.com> wrote:
> > diff --git a/compiler/typecheck_coerce.m b/compiler/typecheck_coerce.m
> > index 6ecfd6e34..e47e0c382 100644
> > --- a/compiler/typecheck_coerce.m
> > +++ b/compiler/typecheck_coerce.m
> > @@ -313,8 +313,12 @@ build_type_param_variance_restrictions_in_ctor_arg_type(TypeTable, CurTypeCtor,
> >                  type_vars_in_types(ArgTypes, TypeVars),
> >                  set.insert_list(TypeVars, !InvariantSet)
> >              ;
> > -                TypeBody = hlds_eqv_type(_),
> > -                unexpected($pred, "hlds_eqv_type")
> > +                TypeBody = hlds_eqv_type(EqvType0),
> > +                hlds_data.get_type_defn_tparams(TypeDefn, TypeParams),
> > +                map.from_corresponding_lists(TypeParams, TypeArgs, TSubst),
> > +                apply_subst_to_type(TSubst, EqvType0, EqvType),
> > +                build_type_param_variance_restrictions_in_ctor_arg_type(TypeTable,
> > +                    CurTypeCtor, CurTypeParams, EqvType, !InvariantSet)
> >              )
> >          else
> >              unexpected($pred, "undefined type")
> 
> Can you please add a comment explaining the purpose of the
> build_type_param_variance_restrictions_in_ctor_arg_type predicate?
> The reason why I didn't tackle this bug myself was mostly the absence
> of info what this predicate was meant to do.

How about this?

+    % Return the set of type parameters of the given TypeCtor that must
+    % remain invariant during type conversion.
+    %
+    % If T (a type parameter in the common base type of a type conversion)
+    % is in the invariant set, then T must be bound to the same type on
+    % both sides of the conversion.
+    %
 :- pred build_type_param_variance_restrictions(type_table::in,
     type_ctor::in, invariant_set::out) is det.

Peter