🤖 Merge PR #46399 array.prototype.flatmap: support ReadonlyArray by @mpartel

* array.prototype.flatmap: support ReadonlyArray

* array.prototype.flatmap: support readonly result from callback

types/array.prototype.flatmap/array.prototype.flatmap-tests.ts

@@ -45,3 +45,12 @@ getPolyfill()(["foo"], word => word.split("")); // $ExpectType string[]
 
 // `shim` installs a flatMap implementation in `Array` prototype and returns it
 shim()(["foo"], word => word.split("")); // $ExpectType string[]
+
+// `ReadonlyArray` is supported
+(["foo"] as ReadonlyArray<string>).flatMap(word => word.split("")); // $ExpectType string[]
+
+// Readonly result from callback is supported
+flatMap([[1], [2]], a => a as ReadonlyArray<number>); // $ExpectType number[]
+flatMap([[1], [2]] as ReadonlyArray<ReadonlyArray<number>>, a => a); // $ExpectType number[]
+([[1], [2]]).flatMap(a => a as ReadonlyArray<number>); // $ExpectType number[]
+([[1], [2]] as ReadonlyArray<ReadonlyArray<number>>).flatMap(a => a); // $ExpectType number[]

types/array.prototype.flatmap/auto.d.ts

@@ -1,6 +1,13 @@
 interface Array<T> {
     flatMap<U, R extends object | undefined = undefined>(
-        fn: (this: R, x: T, index: number, array: this) => U[],
+        fn: (this: R, x: T, index: number, array: this) => ReadonlyArray<U>,
+        thisArg?: R
+    ): U[];
+}
+
+interface ReadonlyArray<T> {
+    flatMap<U, R extends object | undefined = undefined>(
+        fn: (this: R, x: T, index: number, array: this) => ReadonlyArray<U>,
         thisArg?: R
     ): U[];
 }

types/array.prototype.flatmap/implementation.d.ts

@@ -1,7 +1,7 @@
 // This is the same type as the callable signature in `FlatMap` in `index.d.ts`.
 declare function flatMap<A, B, T extends object | undefined = undefined>(
     xs: ReadonlyArray<A>,
-    fn: (this: T, x: A, index: number, array: A[]) => B[],
+    fn: (this: T, x: A, index: number, array: A[]) => ReadonlyArray<B>,
     thisArg?: T
 ): B[];
 export = flatMap;

types/array.prototype.flatmap/index.d.ts

@@ -10,7 +10,7 @@ import flatMapImpl = require("./implementation");
 interface FlatMap {
     <A, B, T extends object | undefined = undefined>(
         xs: ReadonlyArray<A>,
-        fn: (this: T, x: A, index: number, array: A[]) => B[],
+        fn: (this: T, x: A, index: number, array: A[]) => ReadonlyArray<B>,
         thisArg?: T
     ): B[];
     getPolyfill(): typeof flatMapImpl;