Bug: [no-unnecessary-type-assertion] assumes better type narrowing than typescript offers about typescript-eslint HOT 2 OPEN

Your evaluation is incorrect - we don't report here because we assume TS is better than it is.
The rule is powered purely by the types that TS reports.

If you inspect the types of prop with and without the assertion you can see that without the assertion TS reports the type of prop to be T extends "y" | "x" but with the assertion instead TS reports the type of prop as "y".

I think that this is a bug with TS itself - a behaviour that changed in (from the looks of it based on testing) in TS v4.3. In v4.2 TS correctly reports both locations as the generic - in v4.3+ it exhibits the above behaviour.
The assertion should not be changing the type of the original variable.

from typescript-eslint.

thanks @bradzacher yup I can see the behaviour change in TS v4.3. I will lodge a bug report with them.

from typescript-eslint.