From af0f173fcc0bf84316e188a0e6d4da4dfe11b021 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Fri, 28 Aug 2026 13:52:34 +0100
Subject: Throw appropriate error if enum variant is not recognised

---
 .../data/transient0/HobgoblinTransient0Data.java   |  6 ++-
 .../javatransient0/JavaTransient0Generator.java    | 46 +++++++++++++++++-----
 2 files changed, 41 insertions(+), 11 deletions(-)

diff --git a/examples/16-transient-0/output/java/src/main/java/org/zwobble/example/types/data/transient0/HobgoblinTransient0Data.java b/examples/16-transient-0/output/java/src/main/java/org/zwobble/example/types/data/transient0/HobgoblinTransient0Data.java
index 0cd25d8..13e5044 100644
--- a/examples/16-transient-0/output/java/src/main/java/org/zwobble/example/types/data/transient0/HobgoblinTransient0Data.java
+++ b/examples/16-transient-0/output/java/src/main/java/org/zwobble/example/types/data/transient0/HobgoblinTransient0Data.java
@@ -184,7 +184,11 @@ public final class HobgoblinTransient0Data {
 
     public static org.zwobble.example.types.data.EnumWithVariants decodeEnumWithVariants(java.io.InputStream inputStream, java.util.List<java.lang.Object> sharedValues) throws java.io.IOException {
         var ordinal = org.zwobble.example.types.transient0.HobgoblinTransient0Builtins.decodeInt32(inputStream, sharedValues);
-        return EnumWithVariants$values[ordinal];
+        if (ordinal >= 0 && ordinal < 3) {
+            return EnumWithVariants$values[ordinal];
+        } else {
+            throw new java.io.IOException("unrecognised variant tag for data.EnumWithVariants: " + ordinal);
+        }
     }
 
     public static void encodeStructWithEnum(org.zwobble.example.types.data.StructWithEnum value, java.io.OutputStream outputStream, java.util.Map<org.zwobble.example.types.transient0.HobgoblinTransient0TaggedSharedValue, java.lang.Long> sharedValues) throws java.io.IOException {
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatransient0/JavaTransient0Generator.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatransient0/JavaTransient0Generator.java
index 3a7f0ff..4197a24 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatransient0/JavaTransient0Generator.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatransient0/JavaTransient0Generator.java
@@ -653,11 +653,30 @@ public class JavaTransient0Generator implements Generator {
                 enumDefinition.type(),
                 new JavaBlock(Stream.concat(
                     generateDecode(ordinal, NativeTypes.INT_32, context).stream(),
-                    // TODO: handle ordinal not in array
-                    Stream.of(new JavaReturn(new JavaArrayAccess(
-                        new JavaRef(enumValues),
-                        new JavaRef(ordinal)
-                    )))
+                    Stream.of(new JavaIfStatement(
+                        new JavaBinaryOperation(
+                            JavaBinaryOperator.LOGICAL_AND,
+                            new JavaBinaryOperation(
+                                JavaBinaryOperator.GREATER_THAN_OR_EQUAL,
+                                new JavaRef(ordinal),
+                                new JavaIntegerLiteral(0)
+                            ),
+                            new JavaBinaryOperation(
+                                JavaBinaryOperator.LESS_THAN,
+                                new JavaRef(ordinal),
+                                new JavaIntegerLiteral(enumDefinition.variants().size())
+                            )
+                        ),
+                        new JavaBlock(List.of(
+                            new JavaReturn(new JavaArrayAccess(
+                                new JavaRef(enumValues),
+                                new JavaRef(ordinal)
+                            ))
+                        )),
+                        new JavaBlock(List.of(
+                            throwUnrecognisedVariantException(enumDefinition.type(), new JavaRef(ordinal))
+                        ))
+                    ))
                 ).toList())
             )
         );
@@ -804,11 +823,7 @@ public class JavaTransient0Generator implements Generator {
         cases.add(new JavaSwitchRule(
             new JavaCaseDefault(),
             new JavaBlock(List.of(
-                throwIoException(new JavaBinaryOperation(
-                    JavaBinaryOperator.ADD,
-                    new JavaStringLiteral("unrecognised variant tag for " + sumDefinition.type().describe() + ": "),
-                    new JavaRef(variantTag)
-                ))
+                throwUnrecognisedVariantException(sumDefinition.type(), new JavaRef(variantTag))
             ))
         ));
 
@@ -1631,6 +1646,17 @@ public class JavaTransient0Generator implements Generator {
         );
     }
 
+    private static JavaThrowStatement throwUnrecognisedVariantException(
+        Type type,
+        JavaExpression tag
+    ) {
+        return throwIoException(new JavaBinaryOperation(
+            JavaBinaryOperator.ADD,
+            new JavaStringLiteral("unrecognised variant tag for " + type.describe() + ": "),
+            tag
+        ));
+    }
+
     private static JavaThrowStatement throwIoException(String errorMessage) {
         return throwIoException(new JavaStringLiteral(errorMessage));
     }
-- 
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