From 59020cc50ac0888a561fe2ff012115ddae6b1e47 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20P=C3=A4rtel?= <martin.partel@gmail.com>
Date: Wed, 29 Jul 2020 09:27:37 +0300
Subject: [PATCH] =?UTF-8?q?=F0=9F=A4=96=20Merge=20PR=20#46399=20array.prot?=
 =?UTF-8?q?otype.flatmap:=20support=20ReadonlyArray=20by=20@mpartel?=
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* array.prototype.flatmap: support ReadonlyArray

* array.prototype.flatmap: support readonly result from callback
---
 .../array.prototype.flatmap-tests.ts                     | 9 +++++++++
 types/array.prototype.flatmap/auto.d.ts                  | 9 ++++++++-
 types/array.prototype.flatmap/implementation.d.ts        | 2 +-
 types/array.prototype.flatmap/index.d.ts                 | 2 +-
 4 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/types/array.prototype.flatmap/array.prototype.flatmap-tests.ts b/types/array.prototype.flatmap/array.prototype.flatmap-tests.ts
index 5236a352ca..e672ebf688 100644
--- a/types/array.prototype.flatmap/array.prototype.flatmap-tests.ts
+++ b/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[]
diff --git a/types/array.prototype.flatmap/auto.d.ts b/types/array.prototype.flatmap/auto.d.ts
index 43177fb3ad..063e6ed800 100644
--- a/types/array.prototype.flatmap/auto.d.ts
+++ b/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[];
 }
diff --git a/types/array.prototype.flatmap/implementation.d.ts b/types/array.prototype.flatmap/implementation.d.ts
index 80ad9f39d0..e2892d17a0 100644
--- a/types/array.prototype.flatmap/implementation.d.ts
+++ b/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;
diff --git a/types/array.prototype.flatmap/index.d.ts b/types/array.prototype.flatmap/index.d.ts
index fe3b53a914..6180272385 100644
--- a/types/array.prototype.flatmap/index.d.ts
+++ b/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;