Improved union/intersection type inference bug?

[bartosz-k]

Just read about this new TypeScript feature and tried to use it in my project, but...

TypeScript Version:

1.8.9

Code

type T = {a:any} | {b:any};

let myObject:{c:T};

function hasA(value:T):value is {a:any}{
  return 'a' in value;
}
if(hasA(myObject.c)){
  myObject.c.a; // <- the error goes from here
}

Expected behavior:

No error.

Actual behavior:

Error:(24, 14) TS2339: Property 'a' does not exist on type '{ a: any; } | { b: any; }'.

Shouldn't it be considered as a bug?

[yortus]

Duplicate of #3812?

That is, the problem is that type guards don't yet work with property accesses (like myObject.c). Your example works if the property access is done before the type guard, which can then use a simple variable:

let c = myObject.c;
if(hasA(c)){
  c.a; // <- no error here anymore
}

[vladima]

@yortus is correct, also #7140 should also address this issue, see section Dotted names in type guards