Support extensions on singleton objects

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

@@ -85,8 +85,7 @@ internal suspend fun executeFunctionDecl(
         }
 
         if (spec.extTypeName != null) {
-            val type = scope[spec.extTypeName]?.value ?: scope.raiseSymbolNotFound("class ${spec.extTypeName} not found")
-            if (type !is ObjClass) scope.raiseClassCastError("${spec.extTypeName} is not the class instance")
+            val type = scope.resolveExtensionReceiverClass(spec.extTypeName)
             scope.addExtension(
                 type,
                 spec.name,
@@ -167,8 +166,7 @@ internal suspend fun executeFunctionDecl(
     val compiledFnBody = annotatedFnBody
 
     spec.extTypeName?.let { typeName ->
-        val type = scope[typeName]?.value ?: scope.raiseSymbolNotFound("class $typeName not found")
-        if (type !is ObjClass) scope.raiseClassCastError("$typeName is not the class instance")
+        val type = scope.resolveExtensionReceiverClass(typeName)
         if (spec.isStatic) {
             type.createClassField(
                 spec.name,

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

@@ -1004,4 +1004,13 @@ open class Scope(
         return rec.value as? net.sergeych.lyng.obj.ObjClass
             ?: raiseClassCastError("Expected class $name, got ${rec.value.objClass.className}")
     }
+
+    internal fun resolveExtensionReceiverClass(name: String): net.sergeych.lyng.obj.ObjClass {
+        val value = get(name)?.value ?: raiseSymbolNotFound("class $name not found")
+        return when (value) {
+            is net.sergeych.lyng.obj.ObjClass -> value
+            is net.sergeych.lyng.obj.ObjInstance -> value.objClass
+            else -> raiseClassCastError("$name is not the class instance")
+        }
+    }
 }

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

@@ -3094,11 +3094,7 @@ class CmdDeclExtProperty(internal val constId: Int, internal val slot: Int) : Cm
     override suspend fun perform(frame: CmdFrame) {
         val decl = frame.fn.constants[constId] as? BytecodeConst.ExtensionPropertyDecl
             ?: error("DECL_EXT_PROPERTY expects ExtensionPropertyDecl at $constId")
-        val type = frame.ensureScope()[decl.extTypeName]?.value
-            ?: frame.ensureScope().raiseSymbolNotFound("class ${decl.extTypeName} not found")
-        if (type !is ObjClass) {
-            frame.ensureScope().raiseClassCastError("${decl.extTypeName} is not the class instance")
-        }
+        val type = frame.ensureScope().resolveExtensionReceiverClass(decl.extTypeName)
         frame.ensureScope().addExtension(
             type,
             decl.property.name,

lynglib/src/commonTest/kotlin/OOTest.kt

@@ -384,6 +384,32 @@ class OOTest {
         }
     }
 
+    @Test
+    fun testObjectSingletonSupportsExtensions() = runTest {
+        val scope = Script.newScope()
+        scope.eval(
+            """
+            object X {
+                fun base() = "base"
+            }
+
+            fun X.decorate(value): String {
+                this.base() + ":" + value.toString()
+            }
+            val X.tag get() = this.base() + ":tag"
+
+            assertEquals("base", X.base())
+            assertEquals("base:42", X.decorate(42))
+            assertEquals("base:ok", X.decorate("ok"))
+            assertEquals("base:tag", X.tag)
+
+            // Wrapper names should be generated for singleton-object receivers too.
+            assertEquals("base:17", __ext__X__decorate(X, 17))
+            assertEquals("base:tag", __ext_get__X__tag(X))
+        """.trimIndent()
+        )
+    }
+
     @Test
     fun testExtensionsAreScopeIsolated() = runTest {
         val scope1 = Script.newScope()