Added overload to ReadonlyArray.prototype.every to make it a type guard if the callback is one.

Fixes microsoft#14963

Author: NaridaL

src/lib/es5.d.ts

@@ -1014,6 +1014,7 @@ interface ReadonlyArray<T> {
       * @param callbackfn A function that accepts up to three arguments. The every method calls the callbackfn function for each element in array1 until the callbackfn returns false, or until the end of 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.
       */
+    every<U extends T>(callbackfn: (value: T, index: number, array: ReadonlyArray<T>) => value is U, thisArg?: any): this is ReadonlyArray<U>;
     every(callbackfn: (value: T, index: number, array: ReadonlyArray<T>) => boolean, thisArg?: any): boolean;
     /**
       * Determines whether the specified callback function returns true for any element of an array.

tests/baselines/reference/arrayEvery.errors.txt

@@ -0,0 +1,21 @@
+tests/cases/compiler/arrayEvery.ts(8,9): error TS2322: Type 'ReadonlyArray<string | number>' is not assignable to type 'ReadonlyArray<number>'.
+  Type 'string | number' is not assignable to type 'number'.
+    Type 'string' is not assignable to type 'number'.
+
+
+==== tests/cases/compiler/arrayEvery.ts (1 errors) ====
+    declare const baseReadonlyArray: ReadonlyArray<number | string>;
+    
+    if (baseReadonlyArray.every<number>((x): x is number => "number" === typeof x)) {
+      const numberReadonlyArray: ReadonlyArray<number> = baseReadonlyArray; // should be ReadonlyArray<number>
+    }
+    
+    if (baseReadonlyArray.every((x): x is number => "number" === typeof x)) {
+      const numberReadonlyArray: ReadonlyArray<number> = baseReadonlyArray; // should be ReadonlyArray<number>
+            ~~~~~~~~~~~~~~~~~~~
+!!! error TS2322: Type 'ReadonlyArray<string | number>' is not assignable to type 'ReadonlyArray<number>'.
+!!! error TS2322:   Type 'string | number' is not assignable to type 'number'.
+!!! error TS2322:     Type 'string' is not assignable to type 'number'.
+    }
+    
+    baseReadonlyArray.every(x => "number" === typeof x && x > 2);

tests/baselines/reference/arrayEvery.js

@@ -0,0 +1,21 @@
+//// [arrayEvery.ts]
+declare const baseReadonlyArray: ReadonlyArray<number | string>;
+
+if (baseReadonlyArray.every<number>((x): x is number => "number" === typeof x)) {
+  const numberReadonlyArray: ReadonlyArray<number> = baseReadonlyArray; // should be ReadonlyArray<number>
+}
+
+if (baseReadonlyArray.every((x): x is number => "number" === typeof x)) {
+  const numberReadonlyArray: ReadonlyArray<number> = baseReadonlyArray; // should be ReadonlyArray<number>
+}
+
+baseReadonlyArray.every(x => "number" === typeof x && x > 2);
+
+//// [arrayEvery.js]
+if (baseReadonlyArray.every(function (x) { return "number" === typeof x; })) {
+    var numberReadonlyArray = baseReadonlyArray; // should be ReadonlyArray<number>
+}
+if (baseReadonlyArray.every(function (x) { return "number" === typeof x; })) {
+    var numberReadonlyArray = baseReadonlyArray; // should be ReadonlyArray<number>
+}
+baseReadonlyArray.every(function (x) { return "number" === typeof x && x > 2; });

tests/cases/compiler/arrayEvery.ts

@@ -0,0 +1,11 @@
+declare const baseReadonlyArray: ReadonlyArray<number | string>;
+
+if (baseReadonlyArray.every<number>((x): x is number => "number" === typeof x)) {
+  const numberReadonlyArray: ReadonlyArray<number> = baseReadonlyArray; // should be ReadonlyArray<number>
+}
+
+if (baseReadonlyArray.every((x): x is number => "number" === typeof x)) {
+  const numberReadonlyArray: ReadonlyArray<number> = baseReadonlyArray; // should be ReadonlyArray<number>
+}
+
+baseReadonlyArray.every(x => "number" === typeof x && x > 2);