Minor: add test for obsolete issue

#KT-8050 Obsolete

compiler/testData/diagnostics/testsWithStdLib/kt8050.kt

@@ -0,0 +1,11 @@
+private class X
+
+private operator fun X?.plus(<!UNUSED_PARAMETER!>p<!>: Int) = X()
+
+class C {
+    private val map = hashMapOf<String, X>()
+
+    fun f() {
+        map<!NO_SET_METHOD!>[""]<!> += 1
+    }
+}