"Inapplicable operator modifier" and "Inapplicable infix modifier" are now errors

compiler/testData/codegen/box/functions/localFunctions/kt4784.kt

@@ -1,6 +1,6 @@
 open class T(var value: Int) {}
 
-operator fun plusAssign(): T {
+fun plusAssign(): T {
 
     operator fun T.plusAssign(s: Int) {
         value += s

compiler/testData/codegen/box/functions/localFunctions/localWithDefault.kt

@@ -1,8 +0,0 @@
-class Z {}
-
-fun box(): String {
-    operator fun Z.plus(s : String, d : String = "K") : String {
-        return s + d
-    }
-    return Z() + "O"
-}