From 65342425fe891ad54b997d8228d72770d02c2441 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sun, 2 Aug 2026 13:58:54 +0100
Subject: Check sum and variant are from the same namespace

---
 .../compiler/typechecker/TypeChecker.java          |  9 ++++++--
 .../SumVariantMustBeInSameNamespaceError.java      | 18 +++++++++++++++
 .../typechecker/TypeCheckerSumDefinitionTests.java | 26 ++++++++++++++++++++++
 3 files changed, 51 insertions(+), 2 deletions(-)
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SumVariantMustBeInSameNamespaceError.java

(limited to 'src')

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 c2eae29..a12677e 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
@@ -291,6 +291,13 @@ public class TypeChecker {
                 throw new SumVariantMustBeStructError(variantValueType, untypedVariant.source());
             }
 
+            if (!variantValueStructType.namespaceName().equals(context.namespaceName())) {
+                throw new SumVariantMustBeInSameNamespaceError(
+                    variantValueStructType.namespaceName(),
+                    untypedVariant.source()
+                );
+            }
+
             var variant = new SumVariant(variants.size(), variantType.value(), variantValueStructType);
             variants.add(variant);
         }
@@ -348,8 +355,6 @@ public class TypeChecker {
         Optional<List<Field>> variantTypeFields,
         TypesInfo typesInfo
     ) {
-        // TODO: check that variant and sum are in the same namespace.
-
         for (var sumTypeField : sumTypeFields) {
             var variantTypeField = variantTypeFields.flatMap(
                 fields -> fields.stream()
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SumVariantMustBeInSameNamespaceError.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SumVariantMustBeInSameNamespaceError.java
new file mode 100644
index 0000000..2210d1b
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SumVariantMustBeInSameNamespaceError.java
@@ -0,0 +1,18 @@
+package org.zwobble.hobgoblin.compiler.typechecker.errors;
+
+import org.zwobble.hobgoblin.compiler.errors.SourceError;
+import org.zwobble.hobgoblin.compiler.sources.Source;
+import org.zwobble.hobgoblin.compiler.types.NamespaceName;
+
+public class SumVariantMustBeInSameNamespaceError extends SourceError {
+    private final NamespaceName namespaceName;
+
+    public SumVariantMustBeInSameNamespaceError(NamespaceName namespaceName, Source source) {
+        super("sum variant must be in same namespace, but was from " + namespaceName, source);
+        this.namespaceName = namespaceName;
+    }
+
+    public NamespaceName namespaceName() {
+        return namespaceName;
+    }
+}
diff --git a/src/test/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSumDefinitionTests.java b/src/test/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSumDefinitionTests.java
index 1120bbe..be9790a 100644
--- a/src/test/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSumDefinitionTests.java
+++ b/src/test/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSumDefinitionTests.java
@@ -9,6 +9,7 @@ import org.zwobble.hobgoblin.compiler.ast.untyped.*;
 import org.zwobble.hobgoblin.compiler.builtins.NativeTypes;
 import org.zwobble.hobgoblin.compiler.sources.NullSource;
 import org.zwobble.hobgoblin.compiler.typechecker.errors.SubtypeIsMissingFieldError;
+import org.zwobble.hobgoblin.compiler.typechecker.errors.SumVariantMustBeInSameNamespaceError;
 import org.zwobble.hobgoblin.compiler.typechecker.errors.SumVariantMustBeStructError;
 import org.zwobble.hobgoblin.compiler.typechecker.errors.UnexpectedTypeError;
 import org.zwobble.hobgoblin.compiler.types.*;
@@ -173,6 +174,31 @@ public class TypeCheckerSumDefinitionTests {
         assertThat(error.variantType(), equalTo(NativeTypes.INT_32));
     }
 
+    @Test
+    public void variantMustBeInSameNamespace() {
+        var untyped = UntypedSumDefinitionNode.arbitrary()
+            .withName("X")
+            .withVariants(List.of(
+                UntypedSumVariantDefinitionNode.arbitrary()
+                    .withType(UntypedArb.typeLevelReference("Variant"))
+                    .build()
+            ))
+            .build();
+        var variantType = new StructType(NamespaceName.of("other"), "Variant");
+        var globalContext = TypeCheckerGlobalContext.initial();
+        globalContext.addNativeTypeConstructor(NativeTypes.BOX);
+        var namespaceContext = globalContext
+            .enterNamespace(NamespaceName.of("a", "b"));
+        namespaceContext.declare("Variant", new TypeLevelValueType(variantType), NullSource.INSTANCE);
+
+        var error = assertThrows(
+            SumVariantMustBeInSameNamespaceError.class,
+            () -> typeCheckNamespaceStatement(untyped, namespaceContext)
+        );
+
+        assertThat(error.namespaceName(), equalTo(NamespaceName.of("other")));
+    }
+
     @Test
     public void fieldsAreTypeChecked() {
         var int32Type = SimpleNativeType.builtin("Int32");
-- 
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