Check variant is valid

2 files changed, 26 insertions, 0 deletions

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index 09a55c08..d38c5ac1 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -1156,6 +1156,14 @@ public class TypeChecker {
 
         var expressionTypeInfo = context.sumTypeInfo(expressionType, untyped.source());
         var variantTypes = expressionTypeInfo.variants();
+
+        for (var typedCase : typedCases) {
+            var caseType = typedCase.caseType().value();
+            if (!variantTypes.contains(caseType)) {
+                throw new UnreachableWhenCaseError(caseType, typedCase.source());
+            }
+        }
+
         var handledVariantTypes = typedCases.stream()
             .map(typedCase -> typedCase.caseType().value())
             .collect(Collectors.toSet());
diff --git a/src/main/java/org/shedlang/compiler/typechecker/errors/UnreachableWhenCaseError.java b/src/main/java/org/shedlang/compiler/typechecker/errors/UnreachableWhenCaseError.java
new file mode 100644
index 00000000..803d22ea
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/typechecker/errors/UnreachableWhenCaseError.java
@@ -0,0 +1,18 @@
+package org.shedlang.compiler.typechecker.errors;
+
+import org.shedlang.compiler.errors.SourceError;
+import org.shedlang.compiler.sources.Source;
+import org.shedlang.compiler.types.Type;
+
+public class UnreachableWhenCaseError extends SourceError {
+    private final Type caseType;
+
+    public UnreachableWhenCaseError(Type caseType, Source source) {
+        super("The case for " + caseType.describe() + " is unreachable", source);
+        this.caseType = caseType;
+    }
+
+    public Type caseType() {
+        return caseType;
+    }
+}