Obsolete KT-1776: Can't resolve members of auto-casted "this" in extension without typing "this" explicitly

#KT-1776 Obsolete

compiler/testData/codegen/box/extensionFunctions/kt1776.kt

@@ -0,0 +1,20 @@
+trait Expr {
+    public fun ttFun() : Int = 12
+}
+
+class Num(val value : Int) : Expr
+
+fun Expr.sometest() : Int {
+    if (this is Num) {
+        value
+        return value
+    }
+    return 0;
+}
+
+
+fun box() : String {
+    if (Num(11).sometest() != 11) return "fail ${Num(11).sometest()}"
+
+    return "OK"
+}