Fix for KT-9855: java.lang.IllegalAccessError when using private package level operator method

#KT-9855 Fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/ExpressionCodegen.java

@@ -3156,7 +3156,7 @@ public class ExpressionCodegen extends KtVisitor<StackValue, StackValue> impleme
 
     private StackValue generateAugmentedAssignment(KtBinaryExpression expression) {
         ResolvedCall<?> resolvedCall = CallUtilKt.getResolvedCallWithAssert(expression, bindingContext);
-        FunctionDescriptor descriptor = (FunctionDescriptor) resolvedCall.getResultingDescriptor();
+        FunctionDescriptor descriptor = accessibleFunctionDescriptor(resolvedCall);
         Callable callable = resolveToCallable(descriptor, false, resolvedCall);
         KtExpression lhs = expression.getLeft();
         Type lhsType = expressionType(lhs);

compiler/testData/codegen/box/private/kt9855.kt

@@ -0,0 +1,15 @@
+class MyString(var content : String)
+
+object Greeter {
+    fun sayHello(name : String): String {
+        var result = MyString(name)
+        result += "K"
+        return result.content
+    }
+}
+
+private operator fun MyString.plus(suffix: String) : MyString =  MyString("${this.content}$suffix")
+
+fun box(): String {
+    return Greeter.sayHello("O")
+}

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -6435,6 +6435,12 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/private/arrayConvention.kt");
             doTest(fileName);
         }
+
+        @TestMetadata("kt9855.kt")
+        public void testKt9855() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/private/kt9855.kt");
+            doTest(fileName);
+        }
     }
 
     @TestMetadata("compiler/testData/codegen/box/privateConstructors")