Type inference error for heterogeneous array

[petersolopov]

Search Terms: Generic, Heterogeneous Array

Code

function shuffleArray<I>(array: I[]): I[] {
  return (
    array
      .map(item => [Math.random(), item])
      .sort((a, b) => a[0] - b[0])
      .map(item => item[1])
  );
}

Expected behavior: no error

Actual behavior: error

Type '(number | I)[]' is not assignable to type 'I[]'.
  Type 'number | I' is not assignable to type 'I'.
    'number | I' is assignable to the constraint of type 'I', but 'I' could be instantiated with a different subtype of constraint '{}'.
      Type 'number' is not assignable to type 'I'.
        'number' is assignable to the constraint of type 'I', but 'I' could be instantiated with a different subtype of constraint '{}'.

Playground Link: http://www.typescriptlang.org/play/#code/GYVwdgxgLglg9mABAZwBYmMANgUwIIBOBAhgJ4A8AkgHwAUxRZAXIpQNoC6AlC+x4gG8AUIkQEcUEASS0RoxAxKk58xADoAtsQAOtGFBwbEAXmqI2AWWJRUakmAAmcDbS4AaRPsPcV8tcjgCKFp6DwAjLhMzYjYABn4AWkQwuJ9VUU0dPQMjU08ctgBGNMQuAG4hAF8gA

P.S Flow works fine

[jablko]

Does adding as const help? e.g.

.map(item => [Math.random(), item] as const)

[petersolopov]

@jablko Yes, thanks! But I still think It should work without this feature

[MartinJohns]

It does, if you're explicit with the types:

.map<[number, I]>(item => [Math.random(), item])

By default array-literals are inferred as arrays, so you return an array that has either a number or a I. Changing this to automatically infer as tuples would very likely be a breaking change. And we already have a way to tell the compiler to infer it as a tuple: as const.

[typescript-bot]

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