1 files changed, 21 insertions, 0 deletions
diff --git a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerWhenTests.java b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerWhenTests.java
index aec35c33..031b20c6 100644
--- a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerWhenTests.java
+++ b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerWhenTests.java
@@ -2,12 +2,14 @@ package org.shedlang.compiler.typechecker;
 
 import org.junit.jupiter.api.Test;
 import org.shedlang.compiler.ast.untyped.Untyped;
+import org.shedlang.compiler.typechecker.errors.WhenExpressionIsNotInstanceOfSumTypeError;
 import org.shedlang.compiler.types.*;
 import org.shedlang.compiler.util.P;
 
 import java.util.List;
 import java.util.Map;
 
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.shedlang.compiler.ast.typed.TypedNodeMatchers.*;
 import static org.zwobble.precisely.AssertThat.assertThat;
 import static org.zwobble.precisely.Matchers.equalTo;
@@ -20,6 +22,25 @@ public class TypeCheckerWhenTests {
     private static final StructType VARIANT_TYPE_2 = Types.struct(NAMESPACE_NAME, "Variant2");
 
     @Test
+    public void whenExpressionIsNotInstanceOfSumTypeThenErrorIsThrown() {
+        var untyped = Untyped.when(
+            Untyped.reference("x"),
+            List.of()
+        );
+        var context = TypeCheckerContext.EMPTY
+            .enterGlobal(P.map(
+                Map.entry("x", Types.STRING)
+            ));
+
+        var error = assertThrows(
+            WhenExpressionIsNotInstanceOfSumTypeError.class,
+            () -> TypeChecker.typeCheckExpression(untyped, context)
+        );
+
+        assertThat(error.actualType(), equalTo(Types.STRING));
+    }
+
+    @Test
     public void whenAllCasesAreHandledThenExpressionTypeChecks() {
         var untyped = Untyped.when(
             Untyped.reference("x"),