From 23640d971b09c8cf931cb59292b40ce96692a5db Mon Sep 17 00:00:00 2001
From: Rhys van der Waerden <rhys.vdw@gmail.com>
Date: Wed, 1 Aug 2018 16:44:53 +1000
Subject: [PATCH] Fix issue with Array#flatMap callback return type

Closes #22685
---
 src/lib/esnext.array.d.ts                     |  4 +--
 tests/baselines/reference/arrayFlatMap.js     | 12 +++++++++
 .../baselines/reference/arrayFlatMap.symbols  | 20 +++++++++++++++
 tests/baselines/reference/arrayFlatMap.types  | 25 +++++++++++++++++++
 tests/cases/compiler/arrayFlatMap.ts          |  6 +++++
 5 files changed, 65 insertions(+), 2 deletions(-)
 create mode 100644 tests/baselines/reference/arrayFlatMap.js
 create mode 100644 tests/baselines/reference/arrayFlatMap.symbols
 create mode 100644 tests/baselines/reference/arrayFlatMap.types
 create mode 100644 tests/cases/compiler/arrayFlatMap.ts

diff --git a/src/lib/esnext.array.d.ts b/src/lib/esnext.array.d.ts
index b602e8cc0e8..33bee0c205e 100644
--- a/src/lib/esnext.array.d.ts
+++ b/src/lib/esnext.array.d.ts
@@ -11,7 +11,7 @@ interface ReadonlyArray<T> {
      * thisArg is omitted, undefined is used as the this value.
      */
     flatMap<U, This = undefined> (
-        callback: (this: This, value: T, index: number, array: T[]) => U|U[],
+        callback: (this: This, value: T, index: number, array: T[]) => U|ReadonlyArray<U>,
         thisArg?: This
     ): U[]
 
@@ -125,7 +125,7 @@ interface Array<T> {
      * thisArg is omitted, undefined is used as the this value.
      */
     flatMap<U, This = undefined> (
-        callback: (this: This, value: T, index: number, array: T[]) => U|U[],
+        callback: (this: This, value: T, index: number, array: T[]) => U|ReadonlyArray<U>,
         thisArg?: This
     ): U[]
 
diff --git a/tests/baselines/reference/arrayFlatMap.js b/tests/baselines/reference/arrayFlatMap.js
new file mode 100644
index 00000000000..3c35cedc1f3
--- /dev/null
+++ b/tests/baselines/reference/arrayFlatMap.js
@@ -0,0 +1,12 @@
+//// [arrayFlatMap.ts]
+const array: number[] = [];
+const readonlyArray: ReadonlyArray<number> = [];
+array.flatMap((): ReadonlyArray<number> => []); // ok
+readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+
+
+//// [arrayFlatMap.js]
+var array = [];
+var readonlyArray = [];
+array.flatMap(function () { return []; }); // ok
+readonlyArray.flatMap(function () { return []; }); // ok
diff --git a/tests/baselines/reference/arrayFlatMap.symbols b/tests/baselines/reference/arrayFlatMap.symbols
new file mode 100644
index 00000000000..0a97341657b
--- /dev/null
+++ b/tests/baselines/reference/arrayFlatMap.symbols
@@ -0,0 +1,20 @@
+=== tests/cases/compiler/arrayFlatMap.ts ===
+const array: number[] = [];
+>array : Symbol(array, Decl(arrayFlatMap.ts, 0, 5))
+
+const readonlyArray: ReadonlyArray<number> = [];
+>readonlyArray : Symbol(readonlyArray, Decl(arrayFlatMap.ts, 1, 5))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2016.array.include.d.ts, --, --), Decl(lib.esnext.array.d.ts, --, --))
+
+array.flatMap((): ReadonlyArray<number> => []); // ok
+>array.flatMap : Symbol(Array.flatMap, Decl(lib.esnext.array.d.ts, --, --))
+>array : Symbol(array, Decl(arrayFlatMap.ts, 0, 5))
+>flatMap : Symbol(Array.flatMap, Decl(lib.esnext.array.d.ts, --, --))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2016.array.include.d.ts, --, --), Decl(lib.esnext.array.d.ts, --, --))
+
+readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+>readonlyArray.flatMap : Symbol(ReadonlyArray.flatMap, Decl(lib.esnext.array.d.ts, --, --))
+>readonlyArray : Symbol(readonlyArray, Decl(arrayFlatMap.ts, 1, 5))
+>flatMap : Symbol(ReadonlyArray.flatMap, Decl(lib.esnext.array.d.ts, --, --))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2016.array.include.d.ts, --, --), Decl(lib.esnext.array.d.ts, --, --))
+
diff --git a/tests/baselines/reference/arrayFlatMap.types b/tests/baselines/reference/arrayFlatMap.types
new file mode 100644
index 00000000000..7eb4d444ecf
--- /dev/null
+++ b/tests/baselines/reference/arrayFlatMap.types
@@ -0,0 +1,25 @@
+=== tests/cases/compiler/arrayFlatMap.ts ===
+const array: number[] = [];
+>array : number[]
+>[] : undefined[]
+
+const readonlyArray: ReadonlyArray<number> = [];
+>readonlyArray : ReadonlyArray<number>
+>[] : undefined[]
+
+array.flatMap((): ReadonlyArray<number> => []); // ok
+>array.flatMap((): ReadonlyArray<number> => []) : number[]
+>array.flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | ReadonlyArray<U>, thisArg?: This) => U[]
+>array : number[]
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | ReadonlyArray<U>, thisArg?: This) => U[]
+>(): ReadonlyArray<number> => [] : () => ReadonlyArray<number>
+>[] : undefined[]
+
+readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+>readonlyArray.flatMap((): ReadonlyArray<number> => []) : number[]
+>readonlyArray.flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | ReadonlyArray<U>, thisArg?: This) => U[]
+>readonlyArray : ReadonlyArray<number>
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | ReadonlyArray<U>, thisArg?: This) => U[]
+>(): ReadonlyArray<number> => [] : () => ReadonlyArray<number>
+>[] : undefined[]
+
diff --git a/tests/cases/compiler/arrayFlatMap.ts b/tests/cases/compiler/arrayFlatMap.ts
new file mode 100644
index 00000000000..dc67bf02490
--- /dev/null
+++ b/tests/cases/compiler/arrayFlatMap.ts
@@ -0,0 +1,6 @@
+// @lib: esnext
+
+const array: number[] = [];
+const readonlyArray: ReadonlyArray<number> = [];
+array.flatMap((): ReadonlyArray<number> => []); // ok
+readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok