Fix unsound Array.flatMap() typings

Author: jablko

src/lib/es2019.array.d.ts

@@ -10,10 +10,11 @@ interface ReadonlyArray<T> {
      * @param thisArg An object to which the this keyword can refer in the callback function. If
      * 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>,
+    flatMap<U, This = undefined, That extends this> (
+        this: readonly T[],
+        callback: (this: This, value: That[number], index: number, array: That) => U,
         thisArg?: This
-    ): U[]
+    ): (U extends readonly (infer V)[] ? V : U)[];
 
 
     /**
@@ -124,10 +125,11 @@ interface Array<T> {
      * @param thisArg An object to which the this keyword can refer in the callback function. If
      * 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>,
+    flatMap<U, This = undefined, That extends this> (
+        this: readonly T[],
+        callback: (this: This, value: That[number], index: number, array: That) => 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, That extends number[]>(this: readonly number[], callback: (this: This, value: That[number], index: number, array: That) => 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, That extends number[]>(this: readonly number[], callback: (this: This, value: That[number], index: number, array: That) => 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, That extends readonly number[]>(this: readonly number[], callback: (this: This, value: That[number], index: number, array: That) => 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, That extends readonly number[]>(this: readonly number[], callback: (this: This, value: That[number], index: number, array: That) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >(): ReadonlyArray<number> => [] : () => readonly number[]
 >[] : undefined[]