KT-62138 add testcases

compiler/testData/diagnostics/tests/operatorsOverloading/plusAssignOnArray.fir.kt

@@ -0,0 +1,29 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class C {
+    operator fun get(i: Int): C = this
+}
+
+operator fun C.plus(a: Any): C = this
+operator fun C.plusAssign(a: Any) {}
+
+class C1 {
+    operator fun get(i: Int): C = C()
+    operator fun set(i: Int, v: C) {}
+}
+
+class C2 {
+    operator fun set(i: Int, v: C2) {}
+    operator fun get(i: Int): C2 = this
+}
+
+operator fun C2.plus(a: Any):C2 = this
+
+fun test() {
+    val c = C()
+    c[0] += ""
+    var c1 = C1()
+    c1[0] <!ASSIGN_OPERATOR_AMBIGUITY!>+=<!> ""
+    var c2 = C2()
+    c2[0] += ""
+}