From 4c2db9bb403c8ffc04eadb6ab262d5137f216000 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sun, 2 Aug 2026 13:54:53 +0100
Subject: Check that sum variant is struct

---
 .../compiler/typechecker/TypeChecker.java          |  8 +++++--
 .../errors/SumVariantMustBeStructError.java        | 18 ++++++++++++++++
 .../typechecker/TypeCheckerSumDefinitionTests.java | 25 ++++++++++++++++++++++
 3 files changed, 49 insertions(+), 2 deletions(-)
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SumVariantMustBeStructError.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 a40bd92..c2eae29 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
@@ -286,8 +286,12 @@ public class TypeChecker {
             } else {
                 variantValueType = variantType.value();
             }
-            // TODO: handle not struct type
-            var variant = new SumVariant(variants.size(), variantType.value(), (StructType) variantValueType);
+
+            if (!(variantValueType instanceof StructType variantValueStructType)) {
+                throw new SumVariantMustBeStructError(variantValueType, untypedVariant.source());
+            }
+
+            var variant = new SumVariant(variants.size(), variantType.value(), variantValueStructType);
             variants.add(variant);
         }
 
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SumVariantMustBeStructError.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SumVariantMustBeStructError.java
new file mode 100644
index 0000000..78235ae
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/errors/SumVariantMustBeStructError.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.Type;
+
+public class SumVariantMustBeStructError extends SourceError {
+    private final Type variantType;
+
+    public SumVariantMustBeStructError(Type variantType, Source source) {
+        super("sum variants must be structs, but was " + variantType.describe(), source);
+        this.variantType = variantType;
+    }
+
+    public Type variantType() {
+        return variantType;
+    }
+}
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 ef67cb5..1120bbe 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.SumVariantMustBeStructError;
 import org.zwobble.hobgoblin.compiler.typechecker.errors.UnexpectedTypeError;
 import org.zwobble.hobgoblin.compiler.types.*;
 
@@ -148,6 +149,30 @@ public class TypeCheckerSumDefinitionTests {
         );
     }
 
+    @Test
+    public void variantMustBeStructType() {
+        var untyped = UntypedSumDefinitionNode.arbitrary()
+            .withName("X")
+            .withVariants(List.of(
+                UntypedSumVariantDefinitionNode.arbitrary()
+                    .withType(UntypedArb.typeLevelReference("Int32"))
+                    .build()
+            ))
+            .build();
+        var globalContext = TypeCheckerGlobalContext.initial();
+        globalContext.addNativeType(NativeTypes.INT_32);
+        globalContext.addNativeTypeConstructor(NativeTypes.BOX);
+        var namespaceContext = globalContext
+            .enterNamespace(NamespaceName.of("a", "b"));
+
+        var error = assertThrows(
+            SumVariantMustBeStructError.class,
+            () -> typeCheckNamespaceStatement(untyped, namespaceContext)
+        );
+
+        assertThat(error.variantType(), equalTo(NativeTypes.INT_32));
+    }
+
     @Test
     public void fieldsAreTypeChecked() {
         var int32Type = SimpleNativeType.builtin("Int32");
-- 
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