Superclass type argument inferred as unknown when it's only used as a method parameter type constraint #63737

🔎 Search Terms

"class" "extends" "constraint" "infer" "unknown"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "infer"

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAECCA8AVAfNA3gWAFDWgWwFMAXACwHsATeAVWgIA8iCA7CmFACjACcBzALmjUAlOgC+2CVmyhIMAEJJUDJqxgIU47NiIBPAA4FoAYTLduBYEWgBeaIoBGZMiAJhmyxizb34AS2YAMwJuaE0AfjDoQWYCADcQgG5oAHoU6CcXN2YdAyMASWZgMwsrW19M13dPVR8EAODQiKiY+KTU9IBXZgBrZjIAdxysIA

💻 Code

class A<T> {
  method<U extends T>(arg: U) {}
}

class B<T> extends A<T> {}

type Correct = B<boolean> extends B<infer T> ? T : never; // boolean
type Incorrect = B<boolean> extends A<infer T> ? T : never; // unknown

🙁 Actual behavior

Incorrect is evaluated to unknown

🙂 Expected behavior

Incorrect is evaluated to boolean

Additional information about the issue

No response