fixed extending extern classes (extension methods)

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt

@@ -1120,7 +1120,16 @@ class Compiler(
         if (implicitType != null) {
             resolutionSink?.referenceMember(name, pos, implicitType)
             val ids = resolveImplicitThisMemberIds(name, pos, implicitType)
-            return ImplicitThisMemberRef(name, pos, ids.fieldId, ids.methodId, implicitType)
+            val inClassContext = codeContexts.any { ctx -> ctx is CodeContext.ClassBody }
+            val currentImplicitType = currentImplicitThisTypeName()
+            val preferredType = when {
+                inClassContext -> implicitType
+                // Extension receiver aliases (extern class name -> host runtime class) can fail strict
+                // variant casts; keep current receiver untyped but preserve non-current receivers.
+                implicitType == currentImplicitType -> null
+                else -> implicitType
+            }
+            return ImplicitThisMemberRef(name, pos, ids.fieldId, ids.methodId, preferredType)
         }
         if (classCtx != null && classCtx.classScopeMembers.contains(name)) {
             resolutionSink?.referenceMember(name, pos, classCtx.name)
@@ -4779,6 +4788,11 @@ class Compiler(
             if (payload != null) return payload
         }
         val receiverClass = resolveReceiverClassForMember(ref.receiver)
+        classMethodReturnClass(receiverClass, ref.name)?.let { return it }
+        inferFieldReturnClass(receiverClass, ref.name)?.let { return it }
+        if (receiverClass != null && isClassScopeCallableMember(receiverClass.className, ref.name)) {
+            resolveClassByName("${receiverClass.className}.${ref.name}")?.let { return it }
+        }
         return inferMethodCallReturnClass(ref.name)
     }
 

lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt

@@ -1067,18 +1067,9 @@ class BytecodeCompiler(
             updateSlotType(dst, SlotType.OBJ)
             return CompiledValue(dst, SlotType.OBJ)
         }
-        if (receiverClass == null && memberName == "negate") {
-            val zeroId = builder.addConst(BytecodeConst.IntVal(0))
-            val zeroSlot = allocSlot()
-            builder.emit(Opcode.CONST_INT, zeroId, zeroSlot)
-            updateSlotType(zeroSlot, SlotType.INT)
-            val obj = ensureObjSlot(value)
-            val dst = allocSlot()
-            builder.emit(Opcode.SUB_OBJ, zeroSlot, obj.slot, dst)
-            updateSlotType(dst, SlotType.OBJ)
-            return CompiledValue(dst, SlotType.OBJ)
-        }
-        if (memberName == "negate" && receiverClass in setOf(ObjInt.type, ObjReal.type)) {
+        if (memberName == "negate" &&
+            (receiverClass == null || isDelegateClass(receiverClass) || receiverClass in setOf(ObjInt.type, ObjReal.type))
+        ) {
             val zeroId = builder.addConst(BytecodeConst.IntVal(0))
             val zeroSlot = allocSlot()
             builder.emit(Opcode.CONST_INT, zeroId, zeroSlot)
@@ -1095,6 +1086,11 @@ class BytecodeCompiler(
         )
     }
 
+    private fun isDelegateClass(receiverClass: ObjClass): Boolean =
+        receiverClass.className == "Delegate" ||
+            receiverClass.className == "LazyDelegate" ||
+            receiverClass.implementingNames.contains("Delegate")
+
     private fun operatorMemberName(op: BinOp): String? = when (op) {
         BinOp.PLUS -> "plus"
         BinOp.MINUS -> "minus"
@@ -1142,11 +1138,7 @@ class BytecodeCompiler(
     ): CompiledValue? {
         val memberName = operatorMemberName(op) ?: return null
         val receiverClass = resolveReceiverClass(leftRef)
-        if (receiverClass == null ||
-            receiverClass.className == "Delegate" ||
-            receiverClass.className == "LazyDelegate" ||
-            receiverClass.implementingNames.contains("Delegate")
-        ) {
+        if (receiverClass == null || isDelegateClass(receiverClass)) {
             val objOpcode = when (op) {
                 BinOp.PLUS -> Opcode.ADD_OBJ
                 BinOp.MINUS -> Opcode.SUB_OBJ
@@ -4821,9 +4813,26 @@ class BytecodeCompiler(
 
     private data class CallArgs(val base: Int, val count: Int, val planId: Int?)
 
-    private fun resolveExtensionCallableSlot(receiverClass: ObjClass, memberName: String): CompiledValue? {
+    private fun extensionReceiverTypeNames(receiverClass: ObjClass): Set<String> {
+        val names = LinkedHashSet<String>()
         for (cls in receiverClass.mro) {
-            val candidate = extensionCallableName(cls.className, memberName)
+            names.add(cls.className)
+            for ((knownName, knownClass) in nameObjClass) {
+                if (knownClass !== cls && knownClass.className != cls.className) continue
+                names.add(knownName)
+                names.add(knownName.substringAfterLast('.'))
+            }
+        }
+        return names
+    }
+
+    private fun resolveExtensionSlotByReceiverNames(
+        receiverClass: ObjClass,
+        memberName: String,
+        wrapperName: (String, String) -> String
+    ): CompiledValue? {
+        for (receiverName in extensionReceiverTypeNames(receiverClass)) {
+            val candidate = wrapperName(receiverName, memberName)
             if (allowedScopeNames != null &&
                 !allowedScopeNames.contains(candidate) &&
                 !localSlotIndexByName.containsKey(candidate)
@@ -4835,32 +4844,39 @@ class BytecodeCompiler(
         return null
     }
 
+    private fun resolveUniqueExtensionWrapperSlot(
+        memberName: String,
+        wrapperPrefix: String
+    ): CompiledValue? {
+        val suffix = "__$memberName"
+        val candidates = LinkedHashSet<String>()
+        for (name in localSlotIndexByName.keys) {
+            if (name.startsWith(wrapperPrefix) && name.endsWith(suffix)) {
+                candidates.add(name)
+            }
+        }
+        for (name in scopeSlotIndexByName.keys) {
+            if (name.startsWith(wrapperPrefix) && name.endsWith(suffix)) {
+                candidates.add(name)
+            }
+        }
+        if (candidates.size != 1) return null
+        return resolveDirectNameSlot(candidates.first())
+    }
+
+    private fun resolveExtensionCallableSlot(receiverClass: ObjClass, memberName: String): CompiledValue? {
+        return resolveExtensionSlotByReceiverNames(receiverClass, memberName, ::extensionCallableName)
+            ?: resolveUniqueExtensionWrapperSlot(memberName, "__ext__")
+    }
+
     private fun resolveExtensionGetterSlot(receiverClass: ObjClass, memberName: String): CompiledValue? {
-        for (cls in receiverClass.mro) {
-            val candidate = extensionPropertyGetterName(cls.className, memberName)
-            if (allowedScopeNames != null &&
-                !allowedScopeNames.contains(candidate) &&
-                !localSlotIndexByName.containsKey(candidate)
-            ) {
-                continue
-            }
-            resolveDirectNameSlot(candidate)?.let { return it }
-        }
-        return null
+        return resolveExtensionSlotByReceiverNames(receiverClass, memberName, ::extensionPropertyGetterName)
+            ?: resolveUniqueExtensionWrapperSlot(memberName, "__ext_get__")
     }
 
     private fun resolveExtensionSetterSlot(receiverClass: ObjClass, memberName: String): CompiledValue? {
-        for (cls in receiverClass.mro) {
-            val candidate = extensionPropertySetterName(cls.className, memberName)
-            if (allowedScopeNames != null &&
-                !allowedScopeNames.contains(candidate) &&
-                !localSlotIndexByName.containsKey(candidate)
-            ) {
-                continue
-            }
-            resolveDirectNameSlot(candidate)?.let { return it }
-        }
-        return null
+        return resolveExtensionSlotByReceiverNames(receiverClass, memberName, ::extensionPropertySetterName)
+            ?: resolveUniqueExtensionWrapperSlot(memberName, "__ext_set__")
     }
 
     private fun compileCallArgsWithReceiver(
@@ -7468,7 +7484,7 @@ class BytecodeCompiler(
         if (targetClass == null) return null
         if (targetClass == ObjDynamic.type) return ObjDynamic.type
         classFieldTypesByName[targetClass.className]?.get(name)?.let { cls ->
-            if (cls.className == "Delegate" || cls.className == "LazyDelegate" || cls.implementingNames.contains("Delegate")) {
+            if (isDelegateClass(cls)) {
                 return null
             }
             return cls
@@ -7555,8 +7571,8 @@ class BytecodeCompiler(
 
     private fun queueExtensionCallableNames(receiverClass: ObjClass, memberName: String) {
         if (!useScopeSlots && globalSlotInfo.isEmpty()) return
-        for (cls in receiverClass.mro) {
-            val name = extensionCallableName(cls.className, memberName)
+        for (receiverName in extensionReceiverTypeNames(receiverClass)) {
+            val name = extensionCallableName(receiverName, memberName)
             if (allowedScopeNames == null || allowedScopeNames.contains(name)) {
                 pendingScopeNameRefs.add(name)
             }
@@ -7565,12 +7581,12 @@ class BytecodeCompiler(
 
     private fun queueExtensionPropertyNames(receiverClass: ObjClass, memberName: String) {
         if (!useScopeSlots && globalSlotInfo.isEmpty()) return
-        for (cls in receiverClass.mro) {
-            val getter = extensionPropertyGetterName(cls.className, memberName)
+        for (receiverName in extensionReceiverTypeNames(receiverClass)) {
+            val getter = extensionPropertyGetterName(receiverName, memberName)
             if (allowedScopeNames == null || allowedScopeNames.contains(getter)) {
                 pendingScopeNameRefs.add(getter)
             }
-            val setter = extensionPropertySetterName(cls.className, memberName)
+            val setter = extensionPropertySetterName(receiverName, memberName)
             if (allowedScopeNames == null || allowedScopeNames.contains(setter)) {
                 pendingScopeNameRefs.add(setter)
             }

lynglib/src/commonTest/kotlin/TypesTest.kt

@@ -16,9 +16,10 @@
  */
 
 import kotlinx.coroutines.test.runTest
-import net.sergeych.lyng.Script
-import net.sergeych.lyng.ScriptError
-import net.sergeych.lyng.eval
+import net.sergeych.lyng.*
+import net.sergeych.lyng.obj.Obj
+import net.sergeych.lyng.obj.ObjClass
+import net.sergeych.lyng.obj.ObjNull
 import kotlin.test.Test
 import kotlin.test.assertFailsWith
 import kotlin.test.assertTrue
@@ -27,33 +28,42 @@ class TypesTest {
 
     @Test
     fun testTypeCollection1() = runTest {
-        eval("""
+        eval(
+            """
            class Point(x: Real, y: Real)
            assert(Point(1,2).x == 1)
            assert(Point(1,2).y == 2)
            assert(Point(1,2) is Point)
-        """.trimIndent())
+        """.trimIndent()
+        )
 
     }
+
     @Test
     fun testTypeCollection2() = runTest {
-        eval("""
+        eval(
+            """
            fun fn1(x: Real, y: Real): Real { x + y }
-        """.trimIndent())
+        """.trimIndent()
+        )
 
     }
+
     @Test
     fun testTypeCollection3() = runTest {
-        eval("""
+        eval(
+            """
            class Test(a: Int) {
               fun fn1(x: Real, y: Real): Real { x + y }
            }
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testExternDeclarations() = runTest {
-        eval("""
+        eval(
+            """
             extern fun foo1(a: String): Void
             assertThrows { foo1("1") }
            class Test(a: Int) {
@@ -69,22 +79,26 @@ class TypesTest {
            }
 //           println("4")
            
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testUserClassCompareTo() = runTest {
-        eval("""
+        eval(
+            """
             class Point(val a,b)
             
             assertEquals(Point(0,1), Point(0,1) )
             assertNotEquals(Point(0,1), Point(1,1) )
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testUserClassCompareTo2() = runTest {
-        eval("""
+        eval(
+            """
             class Point(val a,b) {
                 var c = 0
             }
@@ -98,12 +112,14 @@ class TypesTest {
             assertEquals(p1, p2)
             assertNotEquals(Point(0,1), Point(1,1) )
             assertNotEquals(Point(0,1), p3)
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testNumericInference() = runTest {
-        eval("""
+        eval(
+            """
             val x = 1
             var y = 2.0
             assert( x is Int )
@@ -111,11 +127,14 @@ class TypesTest {
             assert( x + y is Real )
             assert( abs(x+y) is Real )
             assert( abs(x/y) is Real )
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
+
     @Test
     fun testNumericInferenceBug1() = runTest {
-        eval("""
+        eval(
+            """
             fun findSumLimit(f) {
                 var sum = 0.0
                 for( n in 1..100 ) {
@@ -142,12 +161,14 @@ class TypesTest {
             val limit = findSumLimit { n -> 1.0/n/n }
             assert( limit != null )
             println("Result: "+limit)
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testNullableHints() = runTest {
-        eval("""
+        eval(
+            """
             // nullable, without type os Object?
             class N(x=null)
             assertEquals(null, N().x)
@@ -167,7 +188,8 @@ class TypesTest {
 
     @Test
     fun testIsUnionIntersection() = runTest {
-        eval("""
+        eval(
+            """
             class A
             class B
             class C: A, B
@@ -179,7 +201,8 @@ class TypesTest {
             val v = 1
             assert( v is Int | String | Real )
             assert( !(v is String | Bool) )
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
@@ -221,51 +244,64 @@ class TypesTest {
 
     @Test
     fun testListLiteralInferenceForBounds() = runTest {
-        eval("""
+        eval(
+            """
             fun acceptInts<T: Int>(xs: List<T>) { }
             acceptInts([1, 2, 3])
             val base = [1, 2]
             acceptInts([...base, 3])
-        """.trimIndent())
-        eval("""
+        """.trimIndent()
+        )
+        eval(
+            """
             fun acceptReals<T: Real>(xs: List<T>) { }
             acceptReals([1.0, 2.0, 3.0])
             val base = [1.0, 2.0]
             acceptReals([...base, 3.0])
-        """.trimIndent())
+        """.trimIndent()
+        )
         assertFailsWith<net.sergeych.lyng.ScriptError> {
-            eval("""
+            eval(
+                """
                 fun acceptInts<T: Int>(xs: List<T>) { }
                 acceptInts([1, "a"])
-            """.trimIndent())
+            """.trimIndent()
+            )
         }
         assertFailsWith<net.sergeych.lyng.ScriptError> {
-            eval("""
+            eval(
+                """
                 fun acceptReals<T: Real>(xs: List<T>) { }
                 acceptReals([1.0, "a"])
-            """.trimIndent())
+            """.trimIndent()
+            )
         }
     }
 
     @Test
     fun testMapLiteralInferenceForBounds() = runTest {
-        eval("""
+        eval(
+            """
             fun acceptMap<T: Int>(m: Map<String, T>) { }
             acceptMap({ "a": 1, "b": 2 })
             val base = { "a": 1 }
             acceptMap({ ...base, "b": 3 })
-        """.trimIndent())
+        """.trimIndent()
+        )
         assertFailsWith<net.sergeych.lyng.ScriptError> {
-            eval("""
+            eval(
+                """
                 fun acceptMap<T: Int>(m: Map<String, T>) { }
                 acceptMap({ "a": 1, "b": "x" })
-            """.trimIndent())
+            """.trimIndent()
+            )
         }
     }
 
     @Test
     fun testUnionTypeLists() = runTest {
-        eval("""
+        eval(
+            """
 
             fun fMixed<T>(list: List<T>) {
                 println(list)
@@ -282,12 +318,14 @@ class TypesTest {
             }
             fMixed([1, "two", true])
             fInts([1,2,3])
-        """)
+        """
+        )
     }
 
     @Test
     fun testTypeAliases() = runTest {
-        eval("""
+        eval(
+            """
             type Num = Int | Real
             type AB = A & B
             class A
@@ -308,12 +346,14 @@ class TypesTest {
             type IntList<T: Int> = List<T>
             fun accept<T: Int>(xs: IntList<T>) { }
             accept([1,2,3])
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testMultipleReceivers() = runTest {
-        eval("""
+        eval(
+            """
             class R1(shared,r1="r1")
             class R2(shared,r2="r2")
             
@@ -340,69 +380,85 @@ class TypesTest {
                     assertEquals("r1", r1)
                 }
             }
-        """)
+        """
+        )
     }
 
     @Test
     fun testLambdaTypes1() = runTest {
         val scope = Script.newScope()
         // declare: ok
-        scope.eval("""
+        scope.eval(
+            """
             var l1: (Int,String)->String
-        """.trimIndent())
+        """.trimIndent()
+        )
         // this should be Lyng compile time exception
         assertFailsWith<ScriptError> {
-            scope.eval("""
+            scope.eval(
+                """
             fun test() {
                 // compiler should detect that l1 us called with arguments that does not match
                 // declare type (Int,String)->String:
                 l1()
             }
-            """.trimIndent())
+            """.trimIndent()
+            )
         }
     }
 
     @Test
     fun testLambdaTypesEllipsis() = runTest {
         val scope = Script.newScope()
-        scope.eval("""
+        scope.eval(
+            """
             var l2: (Int,Object...,String)->Real
             var l4: (Int,String...,String)->Real
             var l3: (...)->Int
-        """.trimIndent())
+        """.trimIndent()
+        )
         assertFailsWith<ScriptError> {
-            scope.eval("""
+            scope.eval(
+                """
             fun testTooFew() {
                 l2(1)
             }
-            """.trimIndent())
+            """.trimIndent()
+            )
         }
         assertFailsWith<ScriptError> {
-            scope.eval("""
+            scope.eval(
+                """
             fun testWrongHead() {
                 l2("x", "y")
             }
-            """.trimIndent())
+            """.trimIndent()
+            )
         }
         assertFailsWith<ScriptError> {
-            scope.eval("""
+            scope.eval(
+                """
             fun testWrongEllipsis() {
                 l4(1, 2, "x")
             }
-            """.trimIndent())
+            """.trimIndent()
+            )
         }
-        scope.eval("""
+        scope.eval(
+            """
             fun testOk1() { l2(1, "x") }
             fun testOk2() { l2(1, 2, 3, "x") }
             fun testOk3() { l3() }
             fun testOk4() { l3(1, true, "x") }
             fun testOk5() { l4(1, "a", "b", "x") }
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testSetTyped() = runTest {
-        eval("""
+        eval(
+            """
             var s = Set<String>()
             val typed: Set<String> = s
             assertEquals(Set(), typed)
@@ -413,12 +469,14 @@ class TypesTest {
             assertEquals(Set(), s)
             s += ["foo", "bar"]
             assertEquals(Set("foo", "bar"), s)
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testListTyped() = runTest {
-        eval("""
+        eval(
+            """
             var l = List<String>()
             val typed: List<String> = l
             assertEquals(List(), typed)
@@ -430,13 +488,15 @@ class TypesTest {
 
             l += ["foo", "bar"]
             assertEquals(List("foo", "bar"), l)
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testAliasesInGenerics1() = runTest {
         val scope = Script.newScope()
-        scope.eval("""
+        scope.eval(
+            """
             type IntList<T: Int> = List<T>
             type IntMap<K,V> = Map<K,V>
             type IntSet<T: Int> = Set<T>
@@ -459,13 +519,16 @@ class TypesTest {
             assertEquals(Set("tag1", "tag2", Buffer("tag3")), x.tags)
             x.tags += Buffer("tag4")
             assertEquals(Set("tag1", "tag2", Buffer("tag3"), Buffer("tag4")), x.tags)
-        """)
-        scope.eval("""
+        """
+        )
+        scope.eval(
+            """
             assert(x is X)
             x.tags += "42"
             assertEquals(Set("tag1", "tag2", Buffer("tag3"), Buffer("tag4"), "42"), x.tags)
             
-        """.trimIndent())
+        """.trimIndent()
+        )
         // now this must fail becaise element type does not match the declared:
         assertFailsWith<ScriptError> {
             scope.eval(
@@ -479,7 +542,8 @@ class TypesTest {
     @Test
     fun testAliasesInGenericsList1() = runTest {
         val scope = Script.newScope()
-        scope.eval("""
+        scope.eval(
+            """
             import lyng.buffer
             type Tag = String | Buffer
 
@@ -495,12 +559,15 @@ class TypesTest {
             assertEquals(List("tag1", "tag2", Buffer("tag3")), x.tags)
             x.tags += ["tag4", Buffer("tag5")]
             assertEquals(List("tag1", "tag2", Buffer("tag3"), "tag4", Buffer("tag5")), x.tags)
-        """)
-        scope.eval("""
+        """
+        )
+        scope.eval(
+            """
             assert(x is X)
             x.tags += "42"
             assertEquals(List("tag1", "tag2", Buffer("tag3"), "tag4", Buffer("tag5"), "42"), x.tags)
-        """.trimIndent())
+        """.trimIndent()
+        )
         assertFailsWith<ScriptError> {
             scope.eval(
                 """
@@ -512,18 +579,21 @@ class TypesTest {
 
     @Test
     fun testClassName() = runTest {
-        eval("""
+        eval(
+            """
             class X {
                 var x = 1
             }
             assert( X::class is Class)
             assertEquals("Class", X::class.name)
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testGenericTypes() = runTest {
-        eval("""
+        eval(
+            """
             fun t<T>(): String =
                 when(T) {
                     null -> "%s is Null"(T::class.name)
@@ -532,24 +602,28 @@ class TypesTest {
                 }
             assert( Int is Object)
             assertEquals( t<Int>(), "Class is Object")
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testGenericNullableTypePredicate() = runTest {
-        eval("""
+        eval(
+            """
             fun isTypeNullable<T>(x: T): Bool = T is nullable
             type MaybeInt = Int?
             assert(isTypeNullable<Int?>(null))
             assert(!isTypeNullable<Int>(1))
             assert(MaybeInt is nullable)
             assert(!(Int is nullable))
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testWhenNullableTypeCase() = runTest {
-        eval("""
+        eval(
+            """
             fun describe<T>(x: T): String = when(T) {
                 nullable -> "nullable"
                 else -> "non-null"
@@ -562,11 +636,14 @@ class TypesTest {
             assertEquals("non-null", describe<Int>(1))
             assertEquals("nullable", describeIs<Int?>(null))
             assertEquals("non-null", describeIs<Int>(1))
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
-    @Test fun testIndexer() = runTest {
-        eval("""
+    @Test
+    fun testIndexer() = runTest {
+        eval(
+            """
             class Greeter {
                 override fun getAt(name) = "Hello, %s!"(name)
             }
@@ -581,11 +658,14 @@ class TypesTest {
             
             assertEquals("How do you do, Bob?",Polite["Bob"])
             
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
-    @Test fun testIndexer2() = runTest {
-        eval("""
+    @Test
+    fun testIndexer2() = runTest {
+        eval(
+            """
             class Foo(bar)
             
             class Greeter {
@@ -613,17 +693,65 @@ class TypesTest {
             assertEquals("How do you do, Bob?",g2v.bar)
             assertEquals("How do you do, Bob?",Greeter2()["Bob"].bar)
             
-        """.trimIndent())
+        """.trimIndent()
+        )
     }
 
     @Test
     fun testExternGenerics() = runTest {
-        eval("""
+        eval(
+            """
             extern fun f<T>(x: T): T 
             extern class Cell<T> {
                 var value: T
             }
-        """)
+        """
+        )
+    }
+
+    class ObjFoo : Obj() {
+
+        override val objClass = klass
+
+        var value: Obj = ObjNull
+
+        companion object {
+            val klass = object : ObjClass("ObjFoo") {
+                override suspend fun callOn(scope: Scope): Obj {
+                    return ObjFoo()
+                }
+            }.apply {
+                addProperty("bar", {
+                    check(thisObj is ObjFoo)
+                    thisAs<ObjFoo>().value
+                }, {
+                    thisAs<ObjFoo>().value = args.firstAndOnly()
+                })
+            }
+        }
+    }
+
+    @Test
+    fun testExtensionToExternClasses() = runTest {
+        val scope = Script.newScope()
+        scope.addConst("Foo", ObjFoo.klass)
+        scope.eval(
+            """
+            extern class Foo {
+                var bar
+            }
+            
+            fun Foo.foobar() = "foo" + bar
+            
+            val f = Foo()
+            f.bar = 42
+            assert( f is Foo )
+            assertEquals(42, f.bar)
+            
+            assertEquals("foo42", f.foobar())
+            
+        """.trimIndent()
+        )
     }
 
     @Test