Type `[x, x]` is not assignable to type `x[] & { length: 2; }`

Bug Report

This error is probably self-explanatory. Either this is supposed to work and it’s a bug, or I’m misunderstanding something in which case please feel free to close.

Type '(Record<string, any> | null)[] & { length: 2; }' is not assignable to type '[Record<string, any> | null, Record<string, any> | null]'.ts(2322)

Outer function is typed to return the latter. It calls an inner function, typed to return the former. One would assume this should work.

Possibly related: #44309

🔎 Search Terms

array literal length

🕗 Version & Regression Information

TypeScript 4.1.3

💻 Code

async function readObjectVersions
<L extends number, C extends string[] & { length: L }>
(objectPath: string, versions: C):
Promise<(Record<string, any> | null)[] & { length: L }> {
  return versions.map(version => ({ foo: 'bar' }));
}

const version1: string = 'x';
const version2: string = 'y';

async function _readObjectVersions(objectPath: string):
Promise<[ Record<string, any> | null, Record<string, any> | null ]> {
  return await readObjectVersions(objectPath, [version1, version2]);
}

_readObjectVersions('abc');

🙁 Actual behavior

The error as described above.

🙂 Expected behavior

No type error.

提供された TypeScript サンプルで、TypeScript 4.1.3 を使用して readObjectVersions 関数と _readObjectVersions 関数のエラーを再現することから始めます。マップされた配列とその長さの制約が、タプルの戻り値の型に対してどのようにチェックされるかを追跡します。示された呼び出しが報告されたエラーなしで型チェックを通れば完了です。