Check for duplicate cases

2 files changed, 23 insertions, 4 deletions

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index d38c5ac1..d11cf4c0 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -1156,18 +1156,19 @@ public class TypeChecker {
 
         var expressionTypeInfo = context.sumTypeInfo(expressionType, untyped.source());
         var variantTypes = expressionTypeInfo.variants();
+        var handledVariantTypes = new HashSet<Type>();
 
         for (var typedCase : typedCases) {
             var caseType = typedCase.caseType().value();
             if (!variantTypes.contains(caseType)) {
                 throw new UnreachableWhenCaseError(caseType, typedCase.source());
             }
+            if (handledVariantTypes.contains(caseType)) {
+                throw new DuplicateWhenCaseError(caseType, typedCase.source());
+            }
+            handledVariantTypes.add(caseType);
         }
 
-        var handledVariantTypes = typedCases.stream()
-            .map(typedCase -> typedCase.caseType().value())
-            .collect(Collectors.toSet());
-
         for (var variantType : variantTypes) {
             if (!handledVariantTypes.contains(variantType)) {
                 throw new UnhandledVariantError(variantType, untyped.source());
diff --git a/src/main/java/org/shedlang/compiler/typechecker/errors/DuplicateWhenCaseError.java b/src/main/java/org/shedlang/compiler/typechecker/errors/DuplicateWhenCaseError.java
new file mode 100644
index 00000000..6197479e
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/typechecker/errors/DuplicateWhenCaseError.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 DuplicateWhenCaseError extends SourceError {
+    private final Type caseType;
+
+    public DuplicateWhenCaseError(Type caseType, Source source) {
+        super("The case for " + caseType.describe() + " has already been handled", source);
+        this.caseType = caseType;
+    }
+
+    public Type caseType() {
+        return caseType;
+    }
+}