Type inference error when typescript generics and class inheritance are used together-CodePudding

class Animal<T extends string> {
  eat(food: T): void {
    console.log('eating '   food)
  }
}

class Rabbit extends Animal<'carrot'> {}

class Cat extends Animal<'mouse'>{}

type MyPet = Rabbit | Cat
const myPets: Record<string, MyPet> = {
  'aa': new Rabbit(),
  'bb': new Cat()
}

function getPets (name: string): MyPet {
  return myPets[name]
}

const pet = getPets('aa')


// error: Argument of type 'string' is not assignable to parameter of type 'never'.
pet.eat('carrot')
pet.eat('mouse')

Please check the source code [source code replication], The generic method is called by using the joint type of two subclasses. The parameter type is inferred as never. Shouldn't it be carrot | mouse。

CodePudding user response：

The problem is that your code doesn't allow TS to work properly by being not precise enough with some of the type definitions. This code works as expected:

class Animal<T extends string> {
  eat(food: T): void {
    console.log('eating '   food)
  }
}

class Rabbit extends Animal<'carrot'> {}

class Cat extends Animal<'mouse'>{}

type MyPet = Rabbit | Cat
const myPets = {
  'aa': new Rabbit(),
  'bb': new Cat()
}

function getPets <T extends keyof typeof myPets>(name: T): typeof myPets[T] {
  return myPets[name]
}

const pet = getPets('aa')

pet.eat('carrot')
pet.eat('mouse') // error: Argument of type '"mouse"' is not assignable to parameter of type '"carrot"'

See here

In your original code you were using

const myPets: Record<string, MyPet> = {
  'aa': new Rabbit(),
  'bb': new Cat()
}

So TS will lose the information that myPets.aa is a Rabbit. It will trat myPets.aa as type MyPet. And since the types 'mouse' and 'carrot' have nothing in common, you cannot feed that pet.

CodePudding user response：

class Animal<T extends string> {
  eat(food: T): void {
    console.log('eating '   food)
  }
}

class Rabbit extends Animal<'carrot'> {}

class Cat extends Animal<'mouse'>{}

// type MyPet = Rabbit | Cat
const myPets = {
  'aa': new Rabbit(),
  'bb': new Cat()
}

function getPets <T extends keyof typeof myPets>(name: T): typeof myPets[T] {
  return myPets[name]
}

// Neme is not sure
const name = 'aa' as keyof typeof myPets

const pet = getPets(name)

// error: Argument of type 'string' is not assignable to parameter of type 'never'.
pet.eat('carrot')
pet.eat('mouse')

See here