A[] form works, but Array<A> breaks

[johan]

Bug Report

🔎 Search Terms

site:github.com inurl:/microsoft/TypeScript/issues/ Array [] normalization syntactic sugar

🕗 Version & Regression Information

This works in 3.9.7, broke in 4.0.5, and remains broken in Nightly.

• This changed between versions 3.9.7 and 4.0.5

⏯ Playground Link

Playground link with relevant code

💻 Code

type R<C> = [C, ...R<C>[]] | number;      // works, as expected
type S<C> = [C, ...Array<S<C>>] | number; // fails unexpectedly - should work identically

const r: R<"add"> = ["add", 2, 1];
const s: S<"add"> = ["add", 2, 1];

🙁 Actual behavior

The A[] form in the first line works, but the otherwise-identical Array<…> form on the second line breaks with the warning Type alias 'S' circularly references itself.(2456).

🙂 Expected behavior

Both of these should work, and behave identically, as they are merely syntactic sugar for each other.

[MartinJohns]

Related / duplicate: #40298. The exact same behaviour was mentioned in this comment.

[Uda-Titor]

Can I work on this? This is my first time contributing to Typescript.

[andrewbranch]

@ryoheiudagawa1995 I would encourage you to search for issues with the "good first issue" label. This issue was actually the inspiration for #45643 😄. It will be unapproachable for someone not already very familiar with the checker.