Unexpected type infer for function first argument with default value since 5.1

🔎 Search Terms

"type infer first argument"

🕗 Version & Regression Information

• This is a crash
• This changed between versions v5.0.4 and v5.1.6
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240308#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXy1URBgEYAeAFQD4AKRVALngDEpMcYBPK6gSmaUAUKEiwEKdNjwEiJAEy96TVuwycuimgPjChIAB4AHThngYuRhGw7de8ALzxasAOalBAGnhv5zKKhcfI7UPoH6xqbmltZqGlqhTrQAdKluAM7MANqU3gFcALrBDqH5QuVgeOlmiKSOzgC26a7M1TCErvUA5AAWIBAQOF3eGCDV-oHFoQDe8AC+QpWo1T51ToTEZPSkfEIA9HvwR-AAegD8i1VmUPL1GwrbuwfHpxeXyzW3SU0t8G0d3T6AyGU3gswWSxWACM7nItoh5E9Dsdzu9oWtZJt5PREftkUdUWizGBYZtSLRaD9Whh2qhOk5ev1Bl1QeCkS9UZDiV9MQ9Kc1qbT6fBGcCWSEwfN2Si3kA

💻 Code

export type Factory<T> = (arg1: T, arg2: any) => any
export type Factory2<T> = (...args: [T, any]) => any

declare function infer1<T>(fn: Factory<T>): T
declare function infer2<T>(fn: Factory2<T>): T

const f1 = (msg: string = 'hello', test: any) => { }
const a1 = infer1(f1)
//    ^? const a1: string | undefined
const a2 = infer2(f1)
//    ^? const a2: string | undefined

const f2 = (msg: string = 'hello') => { }
const b = infer1(f2)
//    ^? const b: string | undefined
const b1 = infer2(f2)
//    ^? const b1: string | undefined

const c = infer1((msg: string = 'hello') => { })
//    ^? const c: string
const c2 = infer2((msg: string = 'hello') => { })
//    ^? const c2: string | undefined

🙁 Actual behavior

In my code you'll notice that the type of c is not the same as b, but the argument we passing into function infer1 is actually the same.

I've tested this between v5.0.4 and v5.1.6, but the type of c is different:

// v5.0.4
const c: string | undefined

// v5.1.6
const c: string

After checking v5.1 changelog, I'm still not sure what changes have been made between 5.0 to 5.1.

So I decided to submit this issue as a "Bug report".

🙂 Expected behavior

The type of c is supposed to be string | undefined.

Additional information about the issue

No response

Inizia con il TypeScript Playground collegato e riduci gli esempi di inferenza generica forniti, confrontando TypeScript 5.0.4 con 5.1.6. Traccia il comportamento dell'inferenza dei tipi per i primi parametri con valore predefinito e determina se il risultato segnalato debba includere sempre undefined; il lavoro è completato quando il repro minimo corrisponde al tipo previsto senza introdurre regressioni nei casi correlati.