microsoft / microsoft/TypeScript
Generic Tuple Optional Parameter spread errors with Type instantiation is excessively deep and possibly infinite. ts(2589)
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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
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- 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