TypeScript Version: 3.1.0-dev.20180813

Search Terms:
type guard, narrowing, union, optional, overlapping

Code

// --strictNullChecks on, please
declare function guard(w: any): w is { a: string | undefined };

declare const x: { a?: string };
x.a; // string | undefined
if (guard(x)) {
  x.a; // string | undefined
}

declare const y: { a?: string } | { a: string };
y.a // string | undefined
if (guard(y)) {
  y.a; // string ??
}

Expected behavior:
With --strictNullChecks on: after being narrowed by the type guard, both x.a and y.a should be string | undefined.

Actual behavior:
The type guard narrows y to {a: string} for some reason. The {a?: string} constituent is presumably being eliminated from the union, but I'm not sure why.

Playground Link:
Link

Related Issues:
Didn't find anything obvious... maybe #11403 or #17561?

Noticed in a comment in #21732.