From 9c2421d317068021c64a42be9f785e5ec73afd43 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Fri, 26 Jun 2026 16:25:34 +0100
Subject: Treat variant as subtype of sum

---
 .../compiler/typechecker/TypeChecker.java          | 25 ++++++++++++++++------
 .../typechecker/TypeCheckerGlobalContext.java      |  6 ++++--
 .../typechecker/TypeCheckerNamespaceContext.java   |  5 +++++
 .../compiler/typechecker/TypeCheckerSubtyping.java | 14 ++++++++++--
 .../hobgoblin/compiler/typechecker/TypesInfo.java  |  2 ++
 5 files changed, 42 insertions(+), 10 deletions(-)

(limited to 'src/main/java')

diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
index 34c05cb..8b930aa 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
@@ -189,7 +189,8 @@ public class TypeChecker {
                 sumType,
                 sumTypeFields,
                 structType,
-                typeCheckedFieldDefinitions.fields
+                typeCheckedFieldDefinitions.fields,
+                context.toTypesInfo()
             );
         }
 
@@ -265,7 +266,8 @@ public class TypeChecker {
                     sumType,
                     typeCheckedFieldDefinitions.fields,
                     variantType,
-                    context.fieldsOf(variantType)
+                    context.fieldsOf(variantType),
+                    context.toTypesInfo()
                 );
             }
         }
@@ -299,7 +301,8 @@ public class TypeChecker {
         SumType sumType,
         List<Field> sumTypeFields,
         StructType variantType,
-        List<Field> variantTypeFields
+        List<Field> variantTypeFields,
+        TypesInfo typesInfo
     ) {
         for (var sumTypeField : sumTypeFields) {
             var variantTypeField = variantTypeFields.stream()
@@ -310,7 +313,12 @@ public class TypeChecker {
                 throw new SubtypeIsMissingFieldError(sumType, variantType, sumTypeField.name(), sumTypeField.source());
             }
 
-            checkIsSubtype(variantTypeField.get().type(), sumTypeField.type(), variantTypeField.get().source());
+            checkIsSubtype(
+                variantTypeField.get().type(),
+                sumTypeField.type(),
+                variantTypeField.get().source(),
+                typesInfo
+            );
         }
     }
 
@@ -384,8 +392,13 @@ public class TypeChecker {
         return value;
     }
 
-    private static void checkIsSubtype(Type subtype, Type supertype, Source source) {
-        if (!isSubtype(subtype, supertype)) {
+    private static void checkIsSubtype(
+        Type subtype,
+        Type supertype,
+        Source source,
+        TypesInfo typesInfo
+    ) {
+        if (!isSubtype(subtype, supertype, typesInfo)) {
             throw new UnexpectedTypeError(
                 new TypeSet.SingleType(supertype),
                 subtype,
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerGlobalContext.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerGlobalContext.java
index cf61a22..c66c999 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerGlobalContext.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerGlobalContext.java
@@ -1,9 +1,11 @@
 package org.zwobble.hobgoblin.compiler.typechecker;
 
 import org.zwobble.hobgoblin.compiler.types.*;
-import org.zwobble.hobgoblin.compiler.util.ManyToMany;
 
-import java.util.*;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
 
 public class TypeCheckerGlobalContext {
     private final Map<String, Type> nativeTypes = new HashMap<>();
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerNamespaceContext.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerNamespaceContext.java
index 0ffbe66..82c8273 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerNamespaceContext.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerNamespaceContext.java
@@ -34,6 +34,11 @@ public class TypeCheckerNamespaceContext {
         this.typesInfo = typesInfo;
     }
 
+    public TypesInfo toTypesInfo() {
+        return this.typesInfo;
+    }
+
+
     public NamespaceName namespaceName() {
         return this.namespaceName;
     }
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtyping.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtyping.java
index 673d931..335fa8d 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtyping.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtyping.java
@@ -1,12 +1,22 @@
 package org.zwobble.hobgoblin.compiler.typechecker;
 
+import org.zwobble.hobgoblin.compiler.types.SumType;
 import org.zwobble.hobgoblin.compiler.types.Type;
 
 class TypeCheckerSubtyping {
     private TypeCheckerSubtyping() {
     }
 
-    static boolean isSubtype(Type subtype, Type supertype) {
-        return subtype.equals(supertype);
+    static boolean isSubtype(Type subtype, Type supertype, TypesInfo typesInfo) {
+        if (subtype.equals(supertype)) {
+            return true;
+        }
+
+        if (supertype instanceof SumType supertypeSum) {
+            return typesInfo.sumVariants(supertypeSum).stream()
+                .anyMatch(variantType -> isSubtype(variantType, subtype, typesInfo));
+        }
+
+        return false;
     }
 }
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypesInfo.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypesInfo.java
index 4e896dd..814466c 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypesInfo.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypesInfo.java
@@ -5,6 +5,8 @@ import org.zwobble.hobgoblin.compiler.types.*;
 import java.util.List;
 
 public interface TypesInfo {
+    TypesInfo EMPTY = TypesInfoInMemory.empty();
+
     boolean isDefined(StructType type);
     List<Field> fieldsOf(StructType structType);
 
-- 
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