Assigning to Partial<this> inside the child class leads to error

[nieltg]

TypeScript Version: 3.0.0-dev.20180609

Search Terms: polymorphic this, Partial

Code

class A {
  prop1: number
  method1(attr1: Partial<this>) {
  }
}

class B extends A {
  constructor() {
    super()
    this.method1({prop1: 5})  // <-- Not assignable
  }
}

Expected behavior:
No error should occur here. As there is no error in this code below. (append this to the previous snippet)

let a = new A()
a.method1({prop1: 5})   // <-- No error here
let b = new B()
b.method1({prop1: 5})   // <-- Also no error here

A better opinion is available at the next comment.

Actual behavior:

a.ts:10:18 - error TS2345: Argument of type '{ prop1: 5; }' is not assignable to parameter of type 'Partial<this>'.

Playground Link: Playground · TypeScript

Related Issues:
I'm not sure, but this looks similar to me: #22934

Thank you very much.

[nieltg]

Well, I think this one is tricky. This is what I have learned from polymorphic this. Correct me if I wrong.

Working case

The first use-case of polymorphic this type I encountered on the TypeScript documentation here is used to build fluent interfaces. Here is the simplified snippet.

class A {
  method1(): this {
    return this      // <-- Assignable
  }

  method2(): this {
    return new A()   // <-- Not assignable
  }
}

this is assignable with this but not with A instance because there will be a problem when A becomes a parent class of another class.

class B extends A {
  prop1: number

  // method1(): this {  <-- 'this' type becomes B
  //   return this      <-- 'this' instance is B, B is assignable to B
  // }

  // method2(): this {
  //   return new A()   <-- 'this' instance is B, A isn't assignable to B
  //                        (A doesn't have prop1, but B has)
  // }
}

Partial<this> case

Things become a little different with Partial<this> since it makes every single property of this becomes optional.

class C {
  prop1: number

  method1(): Partial<this> {
    return { prop1: 5 }        // <-- Should be assignable
  }
}

In my opinion, assigning to Partial<this> should be allowed because although the subclass of the base class specifies other properties, all of them will be optional.

class D extends C {
  prop2: number

  // method1(): Partial<this> {   <-- 'this' type becomes C
  //   return { prop1: 5 }        <-- Specified object should be assignable to Partial<C>
  //                                  (as prop2 is now optional)
  // }
}

Thank you very much.

[mhegazy]

Duplicate of #13442. this is a generic type that represents the type of the most derived class type only known at instantiation time.

[nieltg]

@mhegazy
I'm sorry I didn't have any idea to search for generic keyword before. Now, I learn that polymorphic this type is tied to the generic type. Thank you very much for the reference.