Type inference changed for callbacks with optional parameters

[h-joo]

🔎 Search Terms

optional, inference, callback

🕗 Version & Regression Information

• Behaviour changed between versions 5.0 and 5.1

⏯ Playground Link

https://www.typescriptlang.org/play?jsx=0&module=1&ts=5.1.3#code/KYDwDg9gTgLgBAMwK4DsDGMCWEWMwGxmCgB4AVAPgAowpgATTNAQyIC44qA3Z-JYDmQA0cTCnqgOKJAFsARsQCUcALwU4ciBHzBmKRYLgBvAL4AoM2hwBneM1V5CxKjQjXgAfimyFUZWrgYKH5FAG4LKxRbDQcEAiIoF0h3LzhpeWIRAH1U9N9-dSCQ0KA

💻 Code

export function filter<T>(predicate: (value: T, index: number) => boolean): T {}
const a = filter((pose?: number) => true);

🙁 Actual behavior

a gets inferred as number

🙂 Expected behavior

a gets inferred as number|undefined (this is the case in 5.0.4)

I'm not certain if this change is caused by a bug or intentional, but I couldn't find anything from the announcement at least that made this change, so rather asking for clarification on whether this is an intended behavior

Additional information about the issue

The original definition of filter comes from rxjs (simplified in the example for reporting purpose) : https://github.com/ReactiveX/rxjs/blob/2947583bb33e97f3db9e6d9f6cea70c62a173060/src/internal/operators/filter.ts#L6

[Andarist]

Changed in 5.1.0-dev.20230317, this is what has been added in this version: diff. So likely the difference comes from #52609 .

I think this is a bug. Perhaps it was always there in inferFromAnnotatedParameters or maybe there is a specific reason why optional parameters behave differently there. I see that the inference source that is used is just getEffectiveTypeAnnotationNode(declaration) there and that the optional marker is ignored. I'll investigate this later more thoroughly.

[fatcerberus]

It’s kind of a gray area, but the current behavior feels more intuitive fwiw - the optional marker suggests the intent is that the callback either receives a number or else nothing at all.