Fix unsound Array.flatMap() typings

Author: jablko

src/lib/es2019.array.d.ts

@@ -11,9 +11,9 @@ 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 | ReadonlyArray<U>,
+        callback: (this: This, value: T, index: number, array: T[]) => U,
         thisArg?: This
-    ): U[]
+    ): (U extends readonly (infer V)[] ? V : U)[];
 
 
     /**
@@ -125,9 +125,9 @@ 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 | ReadonlyArray<U>,
+        callback: (this: This, value: T, index: number, array: T[]) => U,
         thisArg?: This
-    ): U[]
+    ): (U extends readonly (infer V)[] ? V : U)[];
 
     /**
      * Returns a new array with all sub-array elements concatenated into it recursively up to the

tests/baselines/reference/arrayFlatMap.types

@@ -9,17 +9,17 @@ const readonlyArray: ReadonlyArray<number> = [];
 
 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 | readonly U[], thisArg?: This) => U[]
+>array.flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >array : number[]
->flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | readonly U[], thisArg?: This) => U[]
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >(): ReadonlyArray<number> => [] : () => readonly 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 | readonly U[], thisArg?: This) => U[]
+>readonlyArray.flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >readonlyArray : readonly number[]
->flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | readonly U[], thisArg?: This) => U[]
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >(): ReadonlyArray<number> => [] : () => readonly number[]
 >[] : undefined[]
 

tests/baselines/reference/user/TypeScript-Vue-Starter.log

@@ -0,0 +1,9 @@
+Exit Code: 1
+Standard output:
+src/components/Hello.spec.ts(4,1): error TS2593: Cannot find name 'describe'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.
+src/components/Hello.spec.ts(5,3): error TS2593: Cannot find name 'it'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.
+src/components/Hello.spec.ts(14,5): error TS2304: Cannot find name 'expect'.
+
+
+
+Standard error:

tests/baselines/reference/user/TypeScript-WeChat-Starter.log

@@ -1,12 +1,16 @@
 Exit Code: 1
 Standard output:
-../../../../../node_modules/@types/mocha/index.d.ts(2680,13): error TS2403: Subsequent variable declarations must have the same type.  Variable 'beforeEach' must be of type 'Lifecycle', but here has type 'HookFunction'.
-../../../../../node_modules/@types/mocha/index.d.ts(2698,13): error TS2403: Subsequent variable declarations must have the same type.  Variable 'afterEach' must be of type 'Lifecycle', but here has type 'HookFunction'.
-../../../../../node_modules/@types/mocha/index.d.ts(2714,13): error TS2403: Subsequent variable declarations must have the same type.  Variable 'describe' must be of type 'Describe', but here has type 'SuiteFunction'.
-../../../../../node_modules/@types/mocha/index.d.ts(2735,13): error TS2403: Subsequent variable declarations must have the same type.  Variable 'xdescribe' must be of type 'Describe', but here has type 'PendingSuiteFunction'.
-../../../../../node_modules/@types/mocha/index.d.ts(2749,13): error TS2403: Subsequent variable declarations must have the same type.  Variable 'it' must be of type 'It', but here has type 'TestFunction'.
-../../../../../node_modules/@types/mocha/index.d.ts(2763,13): error TS2403: Subsequent variable declarations must have the same type.  Variable 'test' must be of type 'It', but here has type 'TestFunction'.
-../../../../../node_modules/@types/mocha/index.d.ts(2770,13): error TS2403: Subsequent variable declarations must have the same type.  Variable 'xit' must be of type 'It', but here has type 'PendingTestFunction'.
+src/App.test.tsx(5,1): error TS2593: Cannot find name 'it'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.
+src/components/Hello.test.tsx(5,1): error TS2593: Cannot find name 'it'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.
+src/components/Hello.test.tsx(7,3): error TS2304: Cannot find name 'expect'.
+src/components/Hello.test.tsx(10,1): error TS2593: Cannot find name 'it'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.
+src/components/Hello.test.tsx(12,3): error TS2304: Cannot find name 'expect'.
+src/components/Hello.test.tsx(15,1): error TS2593: Cannot find name 'it'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.
+src/components/Hello.test.tsx(17,3): error TS2304: Cannot find name 'expect'.
+src/components/Hello.test.tsx(20,1): error TS2593: Cannot find name 'it'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.
+src/components/Hello.test.tsx(21,3): error TS2304: Cannot find name 'expect'.
+src/components/Hello.test.tsx(26,1): error TS2593: Cannot find name 'it'. Do you need to install type definitions for a test runner? Try `npm i @types/jest` or `npm i @types/mocha` and then add `jest` or `mocha` to the types field in your tsconfig.
+src/components/Hello.test.tsx(27,3): error TS2304: Cannot find name 'expect'.