Merge pull request #10027 from maiermic/type-guards-in-Array.prototype.filter

add Array.prototype.filter signature with type guard

Author: mhegazy

Diff for: src/lib/es5.d.ts

@@ -1060,6 +1060,12 @@ interface ReadonlyArray<T> {
       * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
       */
     map<U>(callbackfn: (value: T, index: number, array: ReadonlyArray<T>) => U, thisArg?: any): U[];
+    /**
+     * Returns the elements of an array that meet the condition specified in a callback function.
+     * @param callbackfn A function that accepts up to three arguments. The filter method calls the callbackfn function one time for each element in the array.
+     * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
+     */
+    filter<S extends T>(callbackfn: (value: T, index: number, array: ReadonlyArray<T>) => value is S, thisArg?: any): S[];
     /**
       * Returns the elements of an array that meet the condition specified in a callback function.
       * @param callbackfn A function that accepts up to three arguments. The filter method calls the callbackfn function one time for each element in the array.