Optional arguments sometimes unable to infer

[unional]

Bug Report

🔎 Search Terms

infer optional arguments parameters.

Here are some issues that might be related:

• Infering type of optional parameter of lambda function is broken when passed to generic function. #33852
• Variadic function argument not getting inferred correctly #45425
• Case for inference failure in T extends F<T> #51377
• Fails to propagate returned generic type as argument of another function #51925
• Inconsistent errors reported between tsc and VSCode (Unions across objects with method overload definitions) #53614

But this case is even simpler.

🕗 Version & Regression Information

4.8.4 - 5.0.4

It is working correctly in 4.7.4

• This changed between versions 4.7.4 and 4.8.4

⏯ Playground Link

Playground link with relevant code

💻 Code

function define<P extends unknown[], Result>(fn: (...args: P) => Result): typeof fn { return fn }


// (optional?: { a: number; } | undefined) => void
const fn1 = define((optional?: { a: number }) => { })

// () => { foo(): void; }
const fn2 = define((optional?: { a: number }) => ({ foo() { } }))

🙁 Actual behavior

// () => { foo(): void; }
const fn2 = define((optional?: { a: number }) => ({ foo() { } }))

🙂 Expected behavior

// (optional?: { a: number; } | undefined) => { foo(): void; }
const fn2 = define((optional?: { a: number }) => ({ foo() { } }))

[jakebailey]

Check nightly; the behavior should be fixed. (On my phone or I'd bisect it.)

[unional]

Yes, confirmed it is fixed in nightly 20130415 20230415

[fatcerberus]

nightly 20130415

2013?
Those TypeScript devs are something else, fixing bugs retroactively