Check all variants are handled

2 files changed, 32 insertions, 2 deletions

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index 3b4df564..09a55c08 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -1140,13 +1140,13 @@ public class TypeChecker {
         var typedExpression = typedExpressionResult.value();
         context = typedExpressionResult.context();
 
-        if (!(typedExpression.type() instanceof SumType)) {
+        if (!(typedExpression.type() instanceof SumType expressionType)) {
             throw new WhenExpressionIsNotInstanceOfSumTypeError(
                 typedExpression.type(),
                 typedExpression.source()
             );
         }
-        
+
         var typedCasesResult = context.mapWithContext(
             untyped.cases(),
             TypeChecker::typeCheckWhenCase
@@ -1154,6 +1154,18 @@ public class TypeChecker {
         var typedCases = typedCasesResult.value();
         context = typedCasesResult.context();
 
+        var expressionTypeInfo = context.sumTypeInfo(expressionType, untyped.source());
+        var variantTypes = expressionTypeInfo.variants();
+        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());
+            }
+        }
+
         var caseBodyTypes = typedCases.stream()
             .map(typedCase -> typedCase.body().type())
             .toList();
diff --git a/src/main/java/org/shedlang/compiler/typechecker/errors/UnhandledVariantError.java b/src/main/java/org/shedlang/compiler/typechecker/errors/UnhandledVariantError.java
new file mode 100644
index 00000000..de30b900
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/typechecker/errors/UnhandledVariantError.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 UnhandledVariantError extends SourceError {
+    private final Type unhandledVariant;
+
+    public UnhandledVariantError(Type unhandledVariant, Source source) {
+        super("The variant " + unhandledVariant.describe() + " is unhandled", source);
+        this.unhandledVariant = unhandledVariant;
+    }
+
+    public Type unhandledVariant() {
+        return unhandledVariant;
+    }
+}