instanceof type guard missing deduction

[abramobagnara]

TypeScript Version: 2.7.0-dev.201xxxxx

Code

class C { }

function f<T extends C>(v: T | string): void {
  if (v instanceof C) {
    const x: T = v;
    console.log(x);
  }
}

Expected behavior:

No compile errors:

Actual behavior:

p.ts(5,11): error TS2322: Type 'string | T' is not assignable to type 'T'.
Type 'string' is not assignable to type 'T'.

[ahejlsberg]

This was an error in previous releases too because C is an empty object type. However, with #19671 we can (and should) get this right.

[abramobagnara]

From last part of comment above it seems you imply that master branch after #19671 merge get this right, but this is not the case. Am I missing something?

I can confirm that if C is not an empty object type Typescript 2.6.1 compile the source without errors.

[ahejlsberg]

@abramobagnara I meant that with the approach we introduced in #19671 we can, but currently don't, get it right even when C is an empty class type (as in your example). You are correct that it used to work when C is not an empty class. Both issues are fixed by #20142 that I just put up.