inferred generic incorrectly widened

[DetachHead]

Bug Report

🔎 Search Terms

inferred generic widened

🕗 Version & Regression Information

4.3.0-beta

⏯ Playground Link

Playground link with relevant code

💻 Code

type Foo<T extends number> = T extends infer R? Bar<R>: never //compile error: Type 'R' does not satisfy the constraint 'number'

type Bar<T extends number> = never

🙁 Actual behavior

R was incorrectly widened to no longer be a number, even though it was inferred from T which is a number

🙂 Expected behavior

no error, as R is a number

[RyanCavanaugh]

I think without loss of generality you could write Bar<T & R> instead here. I'm not sure if we could wire this up easily or not

[DetachHead]

in that case this could potentially be related to #42938?

however many of my use cases for the infer keyword are simply using it to define sort of a "type variable" such that i don't have to repeat code. for example:

type TokenizeFormatDate<Format extends string> = '' extends Format
    ? []
    : LongestString<MatchStart<Format, Keys<FormatMap>>> extends infer Token
    ? Token extends string
        ? [Token, ...TokenizeFormatDate<TrimStart<Format, Length<Token>>>]
        : IndexOf<Format, Keys<FormatMap>> extends infer NextTokenIndex
        ? NextTokenIndex extends -1
            ? [Format]
            : [
                    TrimEnd<
                        Format,
                        // @ts-expect-error NextTokenIndex fails to narrow
                        NextTokenIndex
                    >,
                    ...TokenizeFormatDate<TrimStart<Format, IndexOf<Format, Keys<FormatMap>>>>
              ]
        : never
    : never

so in this case that solution would defeat the purpose, though i realise that's not really the intended use for them