Typescript: Conditional type bug

[Jack-Works]

TypeScript Version: 4.1.0-20201001

Search Terms:

Code

type A<T extends number, Q extends number> =
    [true, true] extends [IsNegative<T>, IsNegative<Q>] ? 'Every thing is ok!' : ['strange', IsNegative<T>, IsNegative<Q>]
type B = A<-2, -2>

// When using this declaration, there is bug
type IsNegative<T extends number> = `${T}` extends `-${string}` ? true : false

// When using this declaration, B is no bug
// type IsNegative<T extends number> = T extends -2 ? true : false

Expected behavior: B is "Every thing is ok!"

Actual behavior: B is ["strange", true, true]

Playground Link: https://www.typescriptlang.org/play?ts=next

Related Issues:

[tadhgmister]

please fill out expected behaviour and actual behaviour, make it super obvious because there are a lot of issues opened here and the team only has so much time.

Curiously this does work:

type IsNegative<T extends number> = `${T}` extends `-${string}` ? true : false
type C<Bool1, Bool2> = [true, true] extends [Bool1,Bool2] ? true : false
// Check is true
type Check = C<IsNegative<-2>, IsNegative<-2>>

[Jack-Works]

@tadhgmister I have updated my code example

[ahejlsberg]

The issue here is that we have no handling of wildcard types in template literal type instantiation (we use wildcard types to form "most permissive instantiations" when determining whether to defer or resolve conditional types). When any placeholder in a template literal is instantiated to a wildcard type, the entire template literal should become the wildcard type. In other words, we need to propagate wildcard types. It's a fairly simple fix, I'll put up a PR.