From 9d508e219ff0c4e160c31fcfc0f4f39a9b2c96d2 Mon Sep 17 00:00:00 2001
From: sergeych <real.sergeych@gmail.com>
Date: Mon, 9 Feb 2026 12:13:33 +0300
Subject: [PATCH] Step 21: union mismatch bytecode throw

---
 bytecode_migration_plan.md                      |  6 +++---
 .../kotlin/net/sergeych/lyng/Compiler.kt        | 15 ++++-----------
 .../commonTest/kotlin/BytecodeRecentOpsTest.kt  | 17 +++++++++++++++++
 3 files changed, 24 insertions(+), 14 deletions(-)

diff --git a/bytecode_migration_plan.md b/bytecode_migration_plan.md
index b6375fd..4f496f2 100644
--- a/bytecode_migration_plan.md
+++ b/bytecode_migration_plan.md
@@ -70,9 +70,9 @@ Goal: migrate the compiler so all values live in frames/bytecode, keeping JVM te
   - [x] Allow `NopStatement` in `containsUnsupportedForBytecode`.
   - [x] Emit `ObjVoid` directly in bytecode for `NopStatement` in statement/value contexts.
   - [x] Add a JVM test that exercises a code path returning `NopStatement` in bytecode (e.g., static class member decl in class body).
-- [ ] Step 21: Union mismatch path in bytecode.
-  - [ ] Replace `UnionTypeMismatchStatement` branch with a bytecode-compilable throw path (no custom `StatementRef` that blocks bytecode).
-  - [ ] Add a JVM test that forces the union mismatch at runtime and asserts the error message.
+- [x] Step 21: Union mismatch path in bytecode.
+  - [x] Replace `UnionTypeMismatchStatement` branch with a bytecode-compilable throw path (no custom `StatementRef` that blocks bytecode).
+  - [x] Add a JVM test that forces the union mismatch at runtime and asserts the error message.
 - [ ] Step 22: Delegated local slots in bytecode.
   - [ ] Support reads/writes/assign-ops/inc/dec for delegated locals (`LocalSlotRef.isDelegated`) in `BytecodeCompiler`.
   - [ ] Remove `containsDelegatedRefs` guard once delegated locals are bytecode-safe.
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt
index 24b54f6..47f8b97 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt
@@ -4341,15 +4341,6 @@ class Compiler(
         }
     }
 
-    private class UnionTypeMismatchStatement(
-        private val message: String,
-        override val pos: Pos
-    ) : Statement() {
-        override suspend fun execute(scope: Scope): Obj {
-            throw ScriptError(pos, message)
-        }
-    }
-
     private fun resolveMemberHostForUnion(typeDecl: TypeDecl, memberName: String, pos: Pos): ObjClass {
         val receiverClass = when (typeDecl) {
             TypeDecl.TypeAny, TypeDecl.TypeNullableAny -> Obj.rootObjectType
@@ -4392,8 +4383,10 @@ class Compiler(
             resolveMemberHostForUnion(option, memberName, pos)
         }
         val unionName = typeDeclName(union)
-        val failStmt = UnionTypeMismatchStatement("value is not $unionName", pos)
-        var current: ObjRef = net.sergeych.lyng.obj.StatementRef(failStmt)
+        val errorMessage = ObjString("value is not $unionName").asReadonly
+        val throwExpr = ExpressionStatement(ConstRef(errorMessage), pos)
+        val throwStmt = ThrowStatement(throwExpr, pos)
+        var current: ObjRef = net.sergeych.lyng.obj.StatementRef(throwStmt)
         for (option in options.asReversed()) {
             val typeRef = net.sergeych.lyng.obj.TypeDeclRef(option, pos)
             val cond = BinaryOpRef(BinOp.IS, left, typeRef)
diff --git a/lynglib/src/commonTest/kotlin/BytecodeRecentOpsTest.kt b/lynglib/src/commonTest/kotlin/BytecodeRecentOpsTest.kt
index 89e9bf6..de960c3 100644
--- a/lynglib/src/commonTest/kotlin/BytecodeRecentOpsTest.kt
+++ b/lynglib/src/commonTest/kotlin/BytecodeRecentOpsTest.kt
@@ -17,6 +17,7 @@
 
 import kotlinx.coroutines.test.runTest
 import net.sergeych.lyng.Compiler
+import net.sergeych.lyng.ExecutionError
 import net.sergeych.lyng.Script
 import net.sergeych.lyng.ScriptError
 import net.sergeych.lyng.Source
@@ -24,6 +25,7 @@ import net.sergeych.lyng.eval
 import net.sergeych.lyng.obj.toInt
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
 import kotlin.test.assertTrue
 
 class BytecodeRecentOpsTest {
@@ -221,6 +223,21 @@ class BytecodeRecentOpsTest {
         )
     }
 
+    @Test
+    fun unionMemberDispatchMismatch() = runTest {
+        val err = assertFailsWith<ExecutionError> {
+            eval(
+                """
+                class A { fun who() = "A" }
+                class B { fun who() = "B" }
+                val x: A | B = 1
+                x.who()
+                """.trimIndent()
+            )
+        }
+        assertTrue(err.message?.contains("value is not A | B") == true)
+    }
+
     @Test
     fun objectReceiverMemberError() = runTest {
         val failed = try {