Nested discriminated union inference

[jeremybparagon]

TypeScript Version: 3.4.0-dev.20190305

Search Terms: nested discriminated union

Code

interface A {
  z: 'a';
}
interface B {
  z: 'b';
}
interface C {
  y: A | B;
}
interface D {
  y: A;
}
const x = (c: C) => {
  if (c.y.z === 'a') {
    const w: A = c.y;
    const v: D = c;
  }
};

Expected behavior: The code compiles successfully.

Actual behavior: The assignment to v fails, with a message that Type 'C' is not assignable to type 'D'.

Playground Link: https://www.typescriptlang.org/play/#src=interface%20A%20%7B%0D%0A%20%20z%3A%20'a'%3B%0D%0A%7D%0D%0Ainterface%20B%20%7B%0D%0A%20%20z%3A%20'b'%3B%0D%0A%7D%0D%0Ainterface%20C%20%7B%0D%0A%20%20y%3A%20A%20%7C%20B%3B%0D%0A%7D%0D%0Ainterface%20D%20%7B%0D%0A%20%20y%3A%20A%3B%0D%0A%7D%0D%0Aconst%20x%20%3D%20(c%3A%20C)%20%3D%3E%20%7B%0D%0A%20%20if%20(c.y.z%20%3D%3D%3D%20'a')%20%7B%0D%0A%20%20%20%20const%20w%3A%20A%20%3D%20c.y%3B%0D%0A%20%20%20%20const%20v%3A%20D%20%3D%20c%3B%0D%0A%20%20%7D%0D%0A%7D%3B%0D%0A

[jack-williams]

I think this is a duplicate of #18758

[jeremybparagon]

Thanks, @jack-williams, I figured someone must have run into this before but couldn't find it. This seems like an even simpler scenario, since here we would really only need the conclusion about y that we already deduce on the previous line to check that the assignment to v should be accepted, as opposed to using the nested discriminant to choose between different possible properties of the parent object c, the kind of thing desired in #18758.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.