AST: Override TypeTransform::transformSubMap() in TypeSubstituter

Author: slavapestov

lib/AST/TypeSubstitution.cpp

@@ -489,6 +489,8 @@ class TypeSubstituter : public TypeTransform<TypeSubstituter> {
     : level(level), IFS(IFS) {}
 
   std::optional<Type> transform(TypeBase *type, TypePosition position);
+
+  SubstitutionMap transformSubstitutionMap(SubstitutionMap subs);
 };
 
 }
@@ -624,6 +626,11 @@ TypeSubstituter::transform(TypeBase *type, TypePosition position) {
                               level);
 }
 
+SubstitutionMap TypeSubstituter::transformSubstitutionMap(SubstitutionMap subs) {
+  // FIXME: Take level into account? Move level down into IFS?
+  return subs.subst(IFS);
+}
+
 Type Type::subst(SubstitutionMap substitutions,
                  SubstOptions options) const {
   InFlightSubstitutionViaSubMap IFS(substitutions, options);