microsoft / microsoft/TypeScript

Generic Tuple Optional Parameter spread errors with Type instantiation is excessively deep and possibly infinite. ts(2589)

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Domain: Conditional Types Possible Improvement
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Description

Bug Report

type StringableParameters<TArgs> 
  = TArgs extends [optionalHead?:(infer OptionalArg), ...optionalRest:infer OptionalRest] //optionalHead? <- this question mark is what's causing error
    ? [(OptionalArg|string)?, ...StringableParameters<OptionalRest>] //ts(2589)
    : []

type Test = StringableParameters<[1,2?,3?]>
// type Test = [(string | 1)?, (string | 2)?, (string | 3)?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, any?, ...any[]]

That question mark TArgs extends [optionalHead?:(infer OptionalArg) allowing parameters to be optional is leading to the infinite nesting with ts2589 error. If I remove that question mark - it compiles, but omit the optional parameters of the tuple

type StringableParameters<TArgs> = TArgs extends [optionalHead:(infer OptionalArg), ...optionalRest:infer OptionalRest]
? [(OptionalArg|string), ...StringableParameters<OptionalRest>]
: []
type Test = StringableParameters<[1, 2?, 3?]>

//type Test = [string | 1] - optional parameters omitted
🔎 Search Terms

Type instantiation is excessively deep and possibly infinite. ts(2589)
Generic Tuple extends Optional argument

🕗 Version & Regression Information

Typescript v5, v4.7.4,...

⏯ Playground Link

Playground link with relevant code

💻 Code
type StringableParameters<TArgs> 
  = TArgs extends [optionalHead?:(infer OptionalArg), ...optionalRest:infer OptionalRest] //optionalHead? <- this question mark is what's causing error
    ? [(OptionalArg|string)?, ...StringableParameters<OptionalRest>] //ts(2589)
    : []

type TestOptional = StringableParameters<[1,2?,3?]>
/* type TestOptional = [(string | 1 | undefined)?, (string | 2 | undefined)?, (string | 3 | undefined)?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, 
unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?,
 unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?,
 unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, unknown?, ... 6 more ...?, any?]
*/

type StringableParametersNoOptional<TArgs> 
  = TArgs extends [optionalHead:(infer OptionalArg), ...optionalRest:infer OptionalRest] //optionalHead? <- this question mark is what's causing error
    ? [(OptionalArg|string), ...StringableParametersNoOptional<OptionalRest>] //ts(2589)
    : []

type TestNoOptional = StringableParametersNoOptional<[1,2?,3?]>
//type TestNoOptional = [string | 1]
🙁 Actual behavior

Type instantiation is excessively deep and possibly infinite.(2589) and if I use ts-ignore, the TestOptional type has excessive amount of unknown optional parameters on the tail

🙂 Expected behavior
type TestOptional = [(string | 1 | undefined)?, (string | 2 | undefined)?, (string | 3 | undefined)?]

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the linked TypeScript Playground reproduction and compare the optional and non-optional tuple conditional types shown in the issue. Investigate why the optional spread recurses into excessive unknown parameters; done means the expected three-element optional tuple is produced without ts(2589).

Written by the indexing model from the issue text.

Assessment

Tech stack
typescript
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
30/100

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