If a union has undefined an initializer will remove null or void from the union as well.

[krisselden]

TypeScript Version: locally with 2.2.1 and in playground

Code

with strictNullChecks option in playground

type Opaque = {} | null | void | undefined;

class VersionOne {
  x: Opaque;
  constructor(x: {} | null | void = null) {
    this.x = x;
  }
}

new VersionOne(<Opaque>2);

class VersionTwo {
  constructor(public x: {} | null | void = null) {
  }
}

new VersionTwo(<Opaque>1);

class VersionThree {
  constructor(public x: Opaque = null) { // null is not assignable to {}
  }
}

new VersionThree(<Opaque>1); // null is not assignable to {} | undefined

class VersionFour {
  constructor(public x: number | void | undefined = 1) {
  }
};

let y = new VersionFour().x + 5; // should error but does not

class VersionFive {
  constructor(public x: number | void = 1) {
  }
};

y = new VersionFive().x + 3; // plus cannot be applied to number | void

Expected behavior:
if undefined is specified explicitly by a union the initializer should not remove the void or null along with undefined.

I would expect VersionThree to behave like VersionTwo and VersionFour to be like VersionFive

Actual behavior:
adding undefined to the type union causes the resulting type to lose null or void.

[chancancode]

This is a regression somewhere between TypeScript 2.1 and 2.2 – the last known good version was 2.1.6

[DanielRosenwasser]

Can you guys take a look at our latest nightly to see if #14498 has fixed the issue?

[krisselden]

@DanielRosenwasser I pretty much can tell by the PR that it will fix this issue, though void wasn't checked as well.

[krisselden]

This was fixed by #14498