For-loop now cannot use extension iterator on a nullable #KT-7428 Fixed

compiler/testData/diagnostics/tests/controlStructures/ForLoopWithExtensionIteratorOnNullable.kt

@@ -0,0 +1,21 @@
+// See also KT-7428
+class Container<K>(val k: K)
+// iterator() must be an extension, otherwise code will not compile
+operator fun <K> Container<K>.iterator(): Iterator<K> = null!!
+
+fun test() {
+    val container: Container<String>? = null
+    // Error
+    container<!UNSAFE_CALL!>.<!>iterator()
+    // for extension iterator, this code compiles, but should not
+    for (s in <!ITERATOR_MISSING!>container<!>) {}
+}
+class OtherContainer<K>(val k: K) {
+    operator fun iterator(): Iterator<K> = null!!
+}
+
+fun test2() {
+    val other: OtherContainer<String>? = null
+    // Error
+    for (s in <!ITERATOR_MISSING!>other<!>) {}
+}