Add tests

Author: jablko

tests/baselines/reference/arrayFlatMap.js

@@ -3,10 +3,18 @@ const array: number[] = [];
 const readonlyArray: ReadonlyArray<number> = [];
 array.flatMap((): ReadonlyArray<number> => []); // ok
 readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+
+// #19535
+
+const [x] = [1].flatMap(undefined as () => number[] | number[][]);
+x == 1;
 
 
 //// [arrayFlatMap.js]
 var array = [];
 var readonlyArray = [];
 array.flatMap(function () { return []; }); // ok
 readonlyArray.flatMap(function () { return []; }); // ok
+// #19535
+var x = [1].flatMap(undefined)[0];
+x == 1;

tests/baselines/reference/arrayFlatMap.symbols

@@ -18,3 +18,14 @@ readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
 >flatMap : Symbol(ReadonlyArray.flatMap, Decl(lib.es2019.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.es2019.array.d.ts, --, --))
 
+// #19535
+
+const [x] = [1].flatMap(undefined as () => number[] | number[][]);
+>x : Symbol(x, Decl(arrayFlatMap.ts, 7, 7))
+>[1].flatMap : Symbol(Array.flatMap, Decl(lib.es2019.array.d.ts, --, --))
+>flatMap : Symbol(Array.flatMap, Decl(lib.es2019.array.d.ts, --, --))
+>undefined : Symbol(undefined)
+
+x == 1;
+>x : Symbol(x, Decl(arrayFlatMap.ts, 7, 7))
+

tests/baselines/reference/arrayFlatMap.types

@@ -23,3 +23,20 @@ readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
 >(): ReadonlyArray<number> => [] : () => readonly number[]
 >[] : undefined[]
 
+// #19535
+
+const [x] = [1].flatMap(undefined as () => number[] | number[][]);
+>x : number | number[]
+>[1].flatMap(undefined as () => number[] | number[][]) : (number | number[])[]
+>[1].flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
+>[1] : number[]
+>1 : 1
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
+>undefined as () => number[] | number[][] : () => number[] | number[][]
+>undefined : undefined
+
+x == 1;
+>x == 1 : boolean
+>x : number | number[]
+>1 : 1
+

tests/cases/compiler/arrayFlatMap.ts

@@ -4,3 +4,8 @@ const array: number[] = [];
 const readonlyArray: ReadonlyArray<number> = [];
 array.flatMap((): ReadonlyArray<number> => []); // ok
 readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+
+// #19535
+
+const [x] = [1].flatMap(undefined as () => number[] | number[][]);
+x == 1;