Bug Report

🔎 Search Terms

• 5.1
• tuple
• array
• spread

Possibly related:

• Comparing a tuple input to a variadic tuple no longer narrows it #54048
• Tuple with optional element and exactOptionalPropertyTypes still returns undefined during iteration #54302

🕗 Version & Regression Information

• This changed between versions v5.0.4 and v5.1.3

⏯ Playground Link

I was unable to reproduce: if you could please steer me in the right direction I'm happy to dig more, but I'm a bit stuck on how to debug this

💻 Code

We have an issue using io-ts. If this is a bug with that library, I'll close this here, but I don't think it is.

The TypeScript compiler is complaining about a number of excerpts from the d.ts file, where the compiler doesn't seem to be making sense.

Here's an example error:

node_modules/io-ts/lib/index.d.ts:1151:24 - error TS2344: Type 'CS' does not satisfy the constraint '[Mixed, Mixed, Mixed[]]'.
  Type '[Mixed, Mixed, ...Mixed[]]' is not assignable to type '[Mixed, Mixed, Mixed[]]'.
    Target allows only 3 element(s) but source may have more.

1151 ) => TaggedUnionC<Tag, CS>

The code it's referring to is here:

export interface TaggedUnionC<Tag extends string, CS extends [Mixed, Mixed, ...Array<Mixed>]>
  extends TaggedUnionType<Tag, CS, TypeOf<CS[number]>, OutputOf<CS[number]>, unknown> {}

export declare const taggedUnion: <Tag extends string, CS extends [Mixed, Mixed, ...Mixed[]]>(
  tag: Tag,
  codecs: CS,
  name?: string
) => TaggedUnionC<Tag, CS>

As you can see, the compiler seems to be collapsing the [Mixed, Mixed, ...Array<Mixed>] to [Mixed, Mixed, Mixed[]], which is incorrect. It then doesn't like CS extending [Mixed, Mixed, ...Mixed[]], which it considers incompatible with its incorrect collapsing.

Weirdly, if I tweak the d.ts file by hand, the error goes away:

- export interface TaggedUnionC<Tag extends string, CS extends [Mixed, Mixed, ...Array<Mixed>]>
+ export interface TaggedUnionC<Tag extends string, CS extends [Mixed, Mixed, ...Mixed[]]>

🙁 Actual behavior

Compilation errors

🙂 Expected behavior

Successful compilation