Update commonSubtype() to produce more specific types

2 files changed, 22 insertions, 12 deletions

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerSubTyping.java b/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerSubTyping.java
index b17758b3..3a7c026b 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerSubTyping.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerSubTyping.java
@@ -81,12 +81,18 @@ public class TypeCheckerSubTyping {
 
                     var lowerBound = Types.BOTTOM;
                     for (var lower : typeParamBounds.lower) {
-                        lowerBound = Types.commonSupertype(List.of(lowerBound, lower), this.typeInfos);
+                        lowerBound = Types.commonSupertype(
+                            List.of(lowerBound, lower),
+                            this.typeInfos
+                        );
                     }
 
                     var upperBound = Types.TOP;
                     for (var upper : typeParamBounds.upper) {
-                        upperBound = Types.commonSubtype(List.of(upperBound, upper));
+                        upperBound = Types.commonSubtype(
+                            List.of(upperBound, upper),
+                            this.typeInfos
+                        );
                     }
 
                     if (!isSubType(lowerBound, upperBound, typeInfos)) {
diff --git a/src/main/java/org/shedlang/compiler/types/Types.java b/src/main/java/org/shedlang/compiler/types/Types.java
index 59d1f96e..2abf4df0 100644
--- a/src/main/java/org/shedlang/compiler/types/Types.java
+++ b/src/main/java/org/shedlang/compiler/types/Types.java
@@ -3,7 +3,6 @@ package org.shedlang.compiler.types;
 import org.shedlang.compiler.typechecker.TypeInfos;
 
 import java.util.List;
-import java.util.stream.Collectors;
 
 import static org.shedlang.compiler.typechecker.TypeCheckerSubTyping.isSubType;
 
@@ -99,17 +98,22 @@ public class Types {
         return TOP;
     }
 
-    public static Type commonSubtype(List<Type> supertypes) {
-        var uniqueSubtypes = supertypes.stream()
-            .filter(subtype -> subtype != Types.TOP)
-            .collect(Collectors.toSet());
+    public static Type commonSubtype(List<Type> supertypes, TypeInfos typeInfos) {
+        var uniqueSupertypes = supertypes.stream()
+            .filter(supertype -> supertype != Types.TOP)
+            .distinct()
+            .toList();
 
-        if (uniqueSubtypes.isEmpty()) {
+        if (uniqueSupertypes.isEmpty()) {
             return TOP;
-        } else if (uniqueSubtypes.size() == 1) {
-            return uniqueSubtypes.iterator().next();
-        } else {
-            return BOTTOM;
         }
+
+        for (var subtype : typeInfos.subtypes(uniqueSupertypes.getFirst())) {
+            if (uniqueSupertypes.stream().allMatch(supertype -> isSubType(subtype, supertype, typeInfos))) {
+                return subtype;
+            }
+        }
+
+        return BOTTOM;
     }
 }