regression with private class members and loosing visibility

[koblas]

Upgrading from TypeScript 2.6.2 to 2.7.1 and got a private variable scoping problem.

TypeScript Version: 2.7.1

Search Terms:

private property does not exist

Code

class Parent {
  pubValue: number;

  private priValue: number;
  private priXXX: number;

  constructor(value: number) {
    this.priValue = value;
    this.pubValue = 7;
  }

  cloneImpl(kind: boolean): Child | OtherChild {
    const copy = kind ? new Child(this.priValue) : new OtherChild(this.priValue);
    copy.pubValue = this.pubValue + 1;
    copy.priXXX = this.priXXX;

    return copy;
  }
}

class Child extends Parent {
  myData: number;

  clone() {
    return this.cloneImpl(true) as Child;
  }
}

class OtherChild extends Parent {
  clone() {
    return this.cloneImpl(false) as OtherChild;
  }
}

const child = new Child(6);

child.clone();

Expected behavior:

In typescript 2.6.2 the code was working and not generating an error. Since cloneImpl() is in the parent class it should have access to it's own private (tried with protected as well) variables.

Actual behavior:

test.ts (16,10): Property 'priXXX' does not exist on type 'Child | OtherChild'. (2339)

Playground Link:

https://www.typescriptlang.org/play/#src=class%20Parent%20%7B%0D%0A%20%20pubValue%3A%20number%3B%0D%0A%0D%0A%20%20protected%20priValue%3A%20number%3B%0D%0A%20%20protected%20priXXX%3A%20number%3B%0D%0A%0D%0A%20%20constructor(value%3A%20number)%20%7B%0D%0A%20%20%20%20this.priValue%20%3D%20value%3B%0D%0A%20%20%20%20this.pubValue%20%3D%207%3B%0D%0A%20%20%7D%0D%0A%0D%0A%20%20cloneImpl(kind%3A%20boolean)%3A%20Child%20%7C%20OtherChild%20%7B%0D%0A%20%20%20%20const%20copy%20%3D%20kind%20%3F%20new%20Child(this.priValue)%20%3A%20new%20OtherChild(this.priValue)%3B%0D%0A%20%20%20%20copy.pubValue%20%3D%20this.pubValue%20%2B%201%3B%0D%0A%20%20%20%20copy.priXXX%20%3D%20this.priXXX%3B%0D%0A%0D%0A%20%20%20%20return%20copy%3B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Aclass%20Child%20extends%20Parent%20%7B%0D%0A%20%20myData%3A%20number%3B%0D%0A%0D%0A%20%20clone()%20%7B%0D%0A%20%20%20%20return%20this.cloneImpl(true)%20as%20Child%3B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Aclass%20OtherChild%20extends%20Parent%20%7B%0D%0A%20%20clone()%20%7B%0D%0A%20%20%20%20return%20this.cloneImpl(false)%20as%20OtherChild%3B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Aconst%20child%20%3D%20new%20Child(6)%3B%0D%0A%0D%0Achild.clone()%3B

Related Issues:

None found

[mhegazy]

This is a side effect of #19671. the type copy is now inferred as Child | OtherChild instead of just Child. private properties are not exposed on the union, since it is not always possible to know if they are applicable to the specific use case or not.
A work around would be to use a type assertion as:

 const copy = kind ? new Child(this.priValue) : <Child> new OtherChild(this.priValue);