returning from a for loop breaks narrowing

[SLaks]

TypeScript Version: 3.8.0-dev.20200107 (nightly on Playground

Search Terms: for return loop

Code

function test() {
  for (var x: string | number = 'a'; typeof x === 'string'; x = x.toUpperCase()) {
    return;
  }
}

Expected behavior:
No error, just as if return; were not present

Actual behavior:

The terminating condition fails to narrow the type for the increment clause:

Property 'toUpperCase' does not exist on type 'string | number'.

I suspect this happens because exiting the loop early (especially with break; means that the narrowing doesn't apply after the loop body.
However, that's never true for the increment clause.

Playground Link: link

Related Issues: None

[RyanCavanaugh]

See #26914

[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.