Typescript infer the array type incorrectly

[tomkunzemann]

TypeScript Version: 3.0.1

Search Terms:
type, infer, bug, map, typings, array

Code

let a: Map<string, Map<string, any>>;

const arr = [
  ['outer1', [['inner11', {}], ['inner12', {}]]],
  ['outer2', [['inner21', {}], ['inner22', {}]]],
];

const arr2: Array<[string, Map<string, any>]> = arr.map(([out, a]) => [out, new Map(a)]);

a = new Map(arr2);

Expected behavior:

Typescript should use only the following type for parameter a

{}[][]

Actual behavior:

Typescript infer the type of parameter a to

string | {}[][]

Playground Link:

https://www.typescriptlang.org/play/#src=let%20a%3A%20Map%3Cstring%2C%20Map%3Cstring%2C%20any%3E%3E%3B%0D%0A%0D%0Aconst%20arr%20%3D%20%5B%0D%0A%20%20%5B'outer1'%2C%20%5B%5B'inner11'%2C%20%7B%7D%5D%2C%20%5B'inner12'%2C%20%7B%7D%5D%5D%5D%2C%0D%0A%20%20%5B'outer2'%2C%20%5B%5B'inner21'%2C%20%7B%7D%5D%2C%20%5B'inner22'%2C%20%7B%7D%5D%5D%5D%2C%0D%0A%5D%3B%0D%0A%0D%0Aconst%20arr2%3A%20Array%3C%5Bstring%2C%20Map%3Cstring%2C%20any%3E%5D%3E%20%3D%20arr.map((%5Bout%2C%20a%5D)%20%3D%3E%20%5Bout%2C%20new%20Map(a)%5D)%3B%0D%0A%0D%0Aa%20%3D%20new%20Map(arr2)%3B%0D%0A

[mattmccutchen]

Duplicate of #3369 and #25516. The code currently has to be written:

let a: Map<string, Map<string, any>>;

const arr: [string, [string, {}][]][] = [
  ['outer1', [['inner11', {}], ['inner12', {}]]],
  ['outer2', [['inner21', {}], ['inner22', {}]]],
];

const arr2: Array<[string, Map<string, any>]> =
  arr.map<[string, Map<string, any>]>(([out, a]) => [out, new Map(a)]);

a = new Map(arr2);

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.