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] = [""].flatMap(undefined as () => string[] | string[][]);
+x == "";
 
 
 //// [arrayFlatMap.js]
 var array = [];
 var readonlyArray = [];
 array.flatMap(function () { return []; }); // ok
 readonlyArray.flatMap(function () { return []; }); // ok
+// #19535
+var x = [""].flatMap(undefined)[0];
+x == "";

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] = [""].flatMap(undefined as () => string[] | string[][]);
+>x : Symbol(x, Decl(arrayFlatMap.ts, 7, 7))
+>[""].flatMap : Symbol(Array.flatMap, Decl(lib.es2019.array.d.ts, --, --))
+>flatMap : Symbol(Array.flatMap, Decl(lib.es2019.array.d.ts, --, --))
+>undefined : Symbol(undefined)
+
+x == "";
+>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] = [""].flatMap(undefined as () => string[] | string[][]);
+>x : string | string[]
+>[""].flatMap(undefined as () => string[] | string[][]) : (string | string[])[]
+>[""].flatMap : <U, This = undefined>(callback: (this: This, value: string, index: number, array: string[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
+>[""] : string[]
+>"" : ""
+>flatMap : <U, This = undefined>(callback: (this: This, value: string, index: number, array: string[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
+>undefined as () => string[] | string[][] : () => string[] | string[][]
+>undefined : undefined
+
+x == "";
+>x == "" : boolean
+>x : string | string[]
+>"" : ""
+

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] = [""].flatMap(undefined as () => string[] | string[][]);
+x == "";