Generic constraint extends is confusing #61694
Description
Acknowledgement
- I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.
Comment
Apologies if this has been asked before...
The extends generic constraint doesn't behave like its name suggests. E.g. <B extends A> does not mean "limit values to subclasses of A", but to just A. For example:
class Fruit {
protected readonly name: string = ""
}
class Apple extends Fruit {
name = "apple"
}
function eat<F extends Fruit>(f: F) {}
eat(new Apple()) // compiles, as expected - Apple does extend Fruit
eat(new Fruit()) // compiles, but it shouldn't - Fruit does not extend self
function eat2<F extends Fruit>(fruits: F[]) {
for (const f of fruits) {
// should compile but fails with "Property 'name' is protected...", as if `f` were just `Fruit`
console.log(`eating ${f.name}...`)
}
}
The extends constraint should actually limit values to subclasses.