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[]