[m-rev.] diff: Include supertypes in unused modules computation.

[Peter Wang]

compiler/unused_imports.m:
    Take types that occur in the supertype part of a subtype definition
    into account when computing used modules.

diff --git a/compiler/unused_imports.m b/compiler/unused_imports.m
index 480298924..d857bf497 100644
--- a/compiler/unused_imports.m
+++ b/compiler/unused_imports.m
@@ -437,9 +437,15 @@ type_used_modules(_TypeCtor, TypeDefn, !UsedModules) :-
         DefinedInThisModule = yes,
         Visibility = type_visibility(TypeStatus),
         (
-            TypeBody = hlds_du_type(Ctors, _, _, _, _),
+            TypeBody = hlds_du_type(Ctors, MaybeSuperType, _, _, _),
             list.foldl(ctor_used_modules(Visibility),
-                one_or_more_to_list(Ctors), !UsedModules)
+                one_or_more_to_list(Ctors), !UsedModules),
+            (
+                MaybeSuperType = yes(SuperType),
+                mer_type_used_modules(Visibility, SuperType, !UsedModules)
+            ;
+                MaybeSuperType = no
+            )
         ;
             TypeBody = hlds_eqv_type(EqvType),
             mer_type_used_modules(Visibility, EqvType, !UsedModules)
-- 
2.30.0