NoInfer interfere with Exclude in Exclude<NoInfer<I>, undefined>`

[JeanMeche]

🔎 Search Terms

• NoInfer
• inteference
• Exclude

🕗 Version & Regression Information

This is not a regression, and doesn't work with 5.7 :

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAogHgYwDYFcAmECqA7DAzAS2wjQB4AlAPigF5ZFUNSA5AewEls8IAnCygDRQUuCIWJpKAbgBQMpBGBRsARgBc9ZOiyjxJUthQBbAEa8oAH2G6iJanRH5baKVDduA9B6isA1vMVlACYNeC0MHCcJFmMzHktrKLtaRLFnV3coL2VWJQgGbTQApQBndU1GHSSyEuAeIgBzBMc0iXtUvRdMrO8-YqgSkIrtSNb9AGU6xuabNpSWzoz3bOxcqHzwkn60crDK0c7SABEAQ2BoKwXndquJJcyVtY3KooUlE54eXYKI2f1DUy8ADaAF0ZtUbn8uo88j8tm8fCYAFbfTYHZykADeAF9wWNJPMofdlr1-AjDEgkKj9lCDChKXjOpDqvdsn0gA

💻 Code

type ExcludeUndefined<R> = Exclude<NoInfer<R>, undefined>;

let n1: ExcludeUndefined<number | undefined> = undefined;     // ok
let n2: ExcludeUndefined<Number | undefined> = undefined;     // not excluded
let s1: ExcludeUndefined<string | undefined> = undefined;     // ok
let s2: ExcludeUndefined<String | undefined> = undefined;     // not excluded
let d1: ExcludeUndefined<Date | undefined> = undefined;       // not excluded
let arr1: ExcludeUndefined<number[] | undefined> = undefined; // not excluded
let obj1: ExcludeUndefined<{} | undefined> = undefined;       // ok
let null1: ExcludeUndefined<null | undefined> = undefined;    // ok

🙁 Actual behavior

undefined is not consistantly excluded.

🙂 Expected behavior

undefined should be consistantly excluded.

Additional information about the issue

No response

[RyanCavanaugh]

This isn't a supported way to use NoInfer. Not sure what the intent of this is.

[jcalz]

What if you have a function whose return type involves NoInfer because you don't want it to be inferred from the variable to which you assign it? I mean, if you have a function that looks like declare function foo<T>(⋯): Exclude<NoInfer<T>, undefined> then you could end up with foo<Date | undefined>(⋯) producing one of these things.

[Andarist]

I agree with @jcalz and NoInfering within a return type is actually something I recommended last month to somebody as a fix for their problems. It's a legit pattern.

[JeanMeche]

I noticed that NoInfer<Exclude<R, undefined>> actually works, but does it break the NoInfer behavior ?

[Andarist]

It shuldn't break it. Whatever is wrapped with NoInfer shouldn't be inferred into. It's an early exit in inferFromTypes:

function inferFromTypes(source: Type, target: Type): void {
    if (!couldContainTypeVariables(target) || isNoInferType(target)) {
        return;
    }