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.

Start by reproducing the error with the readObjectVersions and _readObjectVersions functions in the provided TypeScript sample, using TypeScript 4.1.3. Trace how the mapped array and its length constraint are checked against the tuple return type; done means the shown call type-checks without the reported error.