Use original instead of lowered function to retrieve toString-impl

in string concatenation lowering. Otherwise, inliner tries to get
body of already lowered function, failing.
 #KT-48284 Fixed

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/codegen/FirBlackBoxCodegenTestGenerated.java

@@ -19164,6 +19164,12 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/inlineClasses/inlineInCompanion.kt");
         }
 
+        @Test
+        @TestMetadata("inlineToString.kt")
+        public void testInlineToString() throws Exception {
+            runTest("compiler/testData/codegen/box/inlineClasses/inlineToString.kt");
+        }
+
         @Test
         @TestMetadata("iterateOverArrayOfInlineClassValues.kt")
         public void testIterateOverArrayOfInlineClassValues() throws Exception {

compiler/ir/backend.jvm/lower/src/org/jetbrains/kotlin/backend/jvm/lower/JvmStringConcatenationLowering.kt

@@ -16,9 +16,7 @@ import org.jetbrains.kotlin.backend.jvm.JvmBackendContext
 import org.jetbrains.kotlin.backend.jvm.ir.JvmIrBuilder
 import org.jetbrains.kotlin.backend.jvm.ir.createJvmIrBuilder
 import org.jetbrains.kotlin.ir.builders.*
-import org.jetbrains.kotlin.ir.declarations.IrClass
-import org.jetbrains.kotlin.ir.declarations.IrFile
-import org.jetbrains.kotlin.ir.declarations.IrSimpleFunction
+import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.*
 import org.jetbrains.kotlin.ir.expressions.impl.IrStringConcatenationImpl
 import org.jetbrains.kotlin.ir.types.*
@@ -74,6 +72,7 @@ private fun JvmIrBuilder.lowerInlineClassArgument(expression: IrExpression): IrE
     if (InlineClassAbi.unboxType(expression.type) == null)
         return null
     val toStringFunction = expression.type.classOrNull?.owner?.toStringFunction
+        ?.let { (it as? IrAttributeContainer)?.attributeOwnerId as? IrFunction ?: it }
         ?: return null
     val toStringReplacement = backendContext.inlineClassReplacements.getReplacementFunction(toStringFunction)
         ?: return null

compiler/testData/codegen/box/inlineClasses/inlineToString.kt

@@ -0,0 +1,21 @@
+// TARGET_BACKEND: JVM_IR
+// WITH_RUNTIME
+
+import kotlin.jvm.JvmInline
+
+open class Expando {
+    val expansion: Expansion = Expansion()
+}
+
+@JvmInline
+value class Expansion(val map: MutableMap<String, Any?> = mutableMapOf()) {
+    override inline fun toString(): String = "OK"
+}
+
+data class Foo(val i: Int): Expando() {
+    override fun toString(): String {
+        return "$expansion"
+    }
+}
+
+fun box(): String = Foo(0).toString()

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/codegen/IrBlackBoxCodegenTestGenerated.java

@@ -19164,6 +19164,12 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/inlineClasses/inlineInCompanion.kt");
         }
 
+        @Test
+        @TestMetadata("inlineToString.kt")
+        public void testInlineToString() throws Exception {
+            runTest("compiler/testData/codegen/box/inlineClasses/inlineToString.kt");
+        }
+
         @Test
         @TestMetadata("iterateOverArrayOfInlineClassValues.kt")
         public void testIterateOverArrayOfInlineClassValues() throws Exception {