Support interpretation of String's plus method and its extension version

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/interpreter/IrInterpreter.kt

@@ -172,13 +172,18 @@ class IrInterpreter(irModule: IrModuleFragment) {
         }
         val args = stack.getAll().map { it.state }
 
-        val receiverType = descriptor.dispatchReceiverParameter?.type ?: descriptor.extensionReceiverParameter?.type
+        val receiverType = descriptor.dispatchReceiverParameter?.type
         val argsType = listOfNotNull(receiverType) + descriptor.valueParameters.map { it.original.type }
         val argsValues = args.map {
             when (it) {
-                is Complex -> it.getOriginal()
+                is Complex -> when (irFunction.fqNameWhenAvailable?.asString()) {
+                    // must explicitly convert Common to String in String plus method or else will be taken default toString from Common
+                    "kotlin.String.plus" -> stack.apply { interpretToString(it) }.popReturnValue().asString()
+                    else -> it.getOriginal()
+                }
                 is Primitive<*> -> it.value
-                else -> it // lambda can be used in built in calculation, for example, in null check
+                is Lambda -> it // lambda can be used in built in calculation, for example, in null check or toString
+                else -> TODO("unsupported type of argument for builtins calculations: ${it::class.java}")
             }
         }
         val signature = CompileTimeFunction(methodName, argsType.map { it.toString() })
@@ -684,26 +689,31 @@ class IrInterpreter(irModule: IrModuleFragment) {
         val result = StringBuilder()
         expression.arguments.forEach {
             it.interpret().check { executionResult -> return executionResult }
-            result.append(
-                when (val returnValue = stack.popReturnValue()) {
-                    is Primitive<*> -> returnValue.value.toString()
-                    is Wrapper -> returnValue.value.toString()
-                    is Common -> {
-                        val toStringFun = returnValue.getToStringFunction()
-                        stack.newFrame(initPool = mutableListOf(Variable(toStringFun.getReceiver()!!, returnValue))) {
-                            toStringFun.body?.let { toStringFun.interpret() } ?: calculateBuiltIns(toStringFun)
-                        }.check { executionResult -> return executionResult }
-                        stack.popReturnValue().asString()
-                    }
-                    else -> throw InterpreterException("$returnValue cannot be used in StringConcatenation expression")
-                }
-            )
+            interpretToString(stack.popReturnValue()).check { executionResult -> return executionResult }
+            result.append(stack.popReturnValue().asString())
         }
 
         stack.pushReturnValue(result.toString().toState(expression.type))
         return Next
     }
 
+    private suspend fun interpretToString(state: State): ExecutionResult {
+        val result = when (state) {
+            is Primitive<*> -> state.value.toString()
+            is Wrapper -> state.value.toString()
+            is Common -> {
+                val toStringFun = state.getToStringFunction()
+                return stack.newFrame(initPool = mutableListOf(Variable(toStringFun.getReceiver()!!, state))) {
+                    toStringFun.body?.let { toStringFun.interpret() } ?: calculateBuiltIns(toStringFun)
+                }
+            }
+            is Lambda -> state.toString()
+            else -> throw InterpreterException("${state::class.java} cannot be used in StringConcatenation expression")
+        }
+        stack.pushReturnValue(result.toState(irBuiltIns.stringType))
+        return Next
+    }
+
     private fun interpretFunctionExpression(expression: IrFunctionExpression): ExecutionResult {
         val lambda = Lambda(expression.function, expression.type.classOrNull!!.owner)
         if (expression.function.isLocal) lambda.fields.addAll(stack.getAll()) // TODO save only necessary declarations

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/interpreter/Utils.kt

@@ -163,8 +163,8 @@ fun getPrimitiveClass(fqName: String, asObject: Boolean = false): Class<*>? {
     }
 }
 
-fun IrType.getFqName(): String? {
-    return this.classOrNull?.owner?.fqNameWhenAvailable?.asString()
+fun IrType.getFqName(withNullableSymbol: Boolean = false): String? {
+    return this.classOrNull?.owner?.fqNameWhenAvailable?.asString()?.let { if (this.isNullable() && withNullableSymbol) "$it?" else it }
 }
 
 fun IrFunction.getArgsForMethodInvocation(args: List<Variable>): List<Any?> {

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/interpreter/builtins/IrBuiltInsMapGenerated.kt

@@ -470,6 +470,7 @@ val ternaryFunctions = mapOf<CompileTimeFunction, Function3<Any?, Any?, Any?, An
 
 private fun Any.defaultToString(): String {
     return when (this) {
+        is Lambda -> this.toString()
         is State -> "${this.irClass.name}@" + System.identityHashCode(this).toString(16).padStart(8, '0')
         else -> this.toString().replaceAfter("@", System.identityHashCode(this).toString(16).padStart(8, '0'))
     }

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/interpreter/state/Lambda.kt

@@ -6,11 +6,11 @@
 package org.jetbrains.kotlin.backend.common.interpreter.state
 
 import org.jetbrains.kotlin.backend.common.interpreter.equalTo
+import org.jetbrains.kotlin.backend.common.interpreter.getFqName
 import org.jetbrains.kotlin.backend.common.interpreter.stack.Variable
 import org.jetbrains.kotlin.ir.declarations.IrClass
 import org.jetbrains.kotlin.ir.declarations.IrFunction
 import org.jetbrains.kotlin.ir.expressions.IrCall
-import org.jetbrains.kotlin.ir.util.fqNameForIrSerialization
 import org.jetbrains.kotlin.ir.util.nameForIrSerialization
 
 class Lambda(val irFunction: IrFunction, override val irClass: IrClass) : State {
@@ -32,6 +32,9 @@ class Lambda(val irFunction: IrFunction, override val irClass: IrClass) : State
     }
 
     override fun toString(): String {
-        return "Lambda(${irClass.fqNameForIrSerialization})"
+        val receiver = (irFunction.dispatchReceiverParameter?.type ?: irFunction.extensionReceiverParameter?.type)?.getFqName(true)
+        val arguments = irFunction.valueParameters.map { it.type.getFqName(true) }.joinToString(prefix = "(", postfix = ")")
+        val returnType = irFunction.returnType.getFqName(true)
+        return ("$arguments -> $returnType").let { if (receiver != null) "$receiver.$it" else it }
     }
 }

generators/evaluate/GenerateBuiltInsMap.kt

@@ -71,6 +71,7 @@ fun generateMap(): String {
         """
         private fun Any.defaultToString(): String {
             return when (this) {
+                is Lambda -> this.toString()
                 is State -> "${'$'}{this.irClass.name}@" + System.identityHashCode(this).toString(16).padStart(8, '0')
                 else -> this.toString().replaceAfter("@", System.identityHashCode(this).toString(16).padStart(8, '0'))
             }