[flow] type refinement for Array.includes

Summary: It is a common pattern to use `[a, b, c].includes(foo)` instead of `foo === a || foo === b || foo === c`, but today only the second pattern be used for type refinement.

Reviewed By: panagosg7

Differential Revision: D69577322

fbshipit-source-id: 88192132af64e63a6c5ee07104d61b9cf6d3bbe6

lib/core.js

@@ -796,7 +796,7 @@ declare class $ReadOnlyArray<+T> {
      * @param searchElement The element to search for.
      * @param fromIndex The position in this array at which to begin searching for searchElement.
      */
-    includes(searchElement: mixed, fromIndex?: number): boolean;
+    includes(searchElement: mixed, fromIndex?: number): implies searchElement is T;
     /**
      * Returns the index of the first occurrence of a value in an array.
      * @param searchElement The value to locate in the array.

tests/arrays/includes.js

@@ -0,0 +1,5 @@
+declare var x: number;
+
+if (([1, 2, 3] as $ReadOnlyArray<1 | 2 | 3>).includes(x)) {
+  (x as 1 | 2 | 3);
+}

tests/autocomplete_from_h_to_l/autocomplete_from_h_to_l.exp

@@ -2391,7 +2391,7 @@ Flags: --pretty
     },
     {
       "name":"?.includes",
-      "type":"void | ((searchElement: mixed, fromIndex?: number) => boolean)"
+      "type":"void | ((searchElement: mixed, fromIndex?: number) => implies searchElement is T)"
     },
     {
       "name":"?.indexOf",
@@ -2500,7 +2500,7 @@ Flags: --pretty
     },
     {
       "name":"includes",
-      "type":"(searchElement: mixed, fromIndex?: number) => boolean"
+      "type":"(searchElement: mixed, fromIndex?: number) => implies searchElement is T"
     },
     {
       "name":"indexOf",