Fix common supertype when first type is Bottom

1 files changed, 3 insertions, 2 deletions

diff --git a/src/main/java/org/shedlang/compiler/types/Types.java b/src/main/java/org/shedlang/compiler/types/Types.java
index 20c8e0cc..45476a03 100644
--- a/src/main/java/org/shedlang/compiler/types/Types.java
+++ b/src/main/java/org/shedlang/compiler/types/Types.java
@@ -51,12 +51,13 @@ public class Types {
 
     public static <T extends StructuredType> ConstructedType<T> constructedType(
         TypeConstructor<T> constructor,
-        List<Type> args
+        List<? extends Type> args
     ) {
         return new ConstructedType<>(constructor, args);
     }
 
     public static Type commonSupertype(List<Type> subtypes) {
+        // TODO: test this!
         var uniqueSubtypes = subtypes.stream()
             .filter(subtype -> subtype != Types.BOTTOM)
             .collect(Collectors.toSet());
@@ -64,7 +65,7 @@ public class Types {
         if (uniqueSubtypes.isEmpty()) {
             return BOTTOM;
         } else if (uniqueSubtypes.size() == 1) {
-            return subtypes.get(0);
+            return uniqueSubtypes.iterator().next();
         } else {
             return TOP;
         }