Update commonSupertype to handle supertype of sum and its variants

2 files changed, 13 insertions, 8 deletions

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeInfos.java b/src/main/java/org/shedlang/compiler/typechecker/TypeInfos.java
index ff2e0aad..3910b472 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeInfos.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeInfos.java
@@ -131,12 +131,18 @@ public class TypeInfos {
         };
     }
 
-    public Set<SumType> supertypes(Type type) {
+    /**
+     * Get the supertypes of a type, from most specific to least specific.
+     */
+    public Set<Type> supertypes(Type type) {
+        var supertypes = P.set(type);
         if (type instanceof StructType structType) {
-            return this.structSupertypes.getOrDefault(structType, P.orderedSet());
-        } else {
-            return P.set();
+            supertypes = supertypes.plusAll(
+                this.structSupertypes.getOrDefault(structType, P.orderedSet())
+            );
         }
+        supertypes.plus(Types.TOP);
+        return supertypes;
     }
 
     private Optional<SumTypeInfo> simpleSumTypeInfo(SimpleSumType type) {
diff --git a/src/main/java/org/shedlang/compiler/types/Types.java b/src/main/java/org/shedlang/compiler/types/Types.java
index 2e5f2562..59d1f96e 100644
--- a/src/main/java/org/shedlang/compiler/types/Types.java
+++ b/src/main/java/org/shedlang/compiler/types/Types.java
@@ -83,15 +83,14 @@ public class Types {
     public static Type commonSupertype(List<Type> subtypes, TypeInfos typeInfos) {
         var uniqueSubtypes = subtypes.stream()
             .filter(subtype -> subtype != Types.BOTTOM)
-            .collect(Collectors.toSet());
+            .distinct()
+            .toList();
 
         if (uniqueSubtypes.isEmpty()) {
             return BOTTOM;
-        } else if (uniqueSubtypes.size() == 1) {
-            return uniqueSubtypes.iterator().next();
         }
 
-        for (var supertype : typeInfos.supertypes(uniqueSubtypes.iterator().next())) {
+        for (var supertype : typeInfos.supertypes(uniqueSubtypes.getFirst())) {
             if (uniqueSubtypes.stream().allMatch(subtype -> isSubType(subtype, supertype, typeInfos))) {
                 return supertype;
             }