Regression: Tagged types are incorrectly refined

[arcanis]

Bug Report

🔎 Search Terms

refine regression 4.3 refine never

🕗 Version & Regression Information

Worked with 4.2, stopped working with 4.3 beta

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.3.0-dev.20210402#code/C4TwDgpgBAEghgZwBZQLxQcATgSwHYDmUAZFAN4D6FSiSAXFNgK4QC+A3AFCcBmTeAY2A4A9nigEIweMgAyEQsCQAKGsgYykASgZ4mAWwBGELOU5QLUAPRWoAQSjHMUCDx4iswKAKQQBAayh8b0RoESZTPkFhMSCEKCYECAATRxAoSBEwABtoAHdcYUIoACkAZXNLHB4oZVBMmrUUAEJUdAADTFxCdq1Ky0YkLBE8qDwIUYBRLGGsZXaAFSQceOSRCAQ8AHIvbJERQOycf2g4KCbern6oLCkI8SaAOlzFJC5WTiA

💻 Code

type Hash = string & {__hash: true};

function getHashLength(hash: Hash): number {
 // A best effort check in case our function is used by people writing JS
 if (typeof hash !== `string`)
 throw new Error(`This doesn't look like a hash`);

 return hash.length;
}

🙁 Actual behavior

The hash variable is incorrectly refined to never.

🙂 Expected behavior

The hash variable should remain Hash, since the typeof check is coherent with Hash.

[arcanis]

An ugly workaround if you don't use Prettier is to do (note the extra parentheses):

if ((typeof p) !== `string`)
 throw errors.EBADF(`read`);

[ahejlsberg]

Hmm, yes this is an effect of #43131. The fixes to getTypeFacts are in principle correct, except that here we think that typeof is both "string" and "object", which is impossible and therefore gets us to never. In tagged primitive types like this we should really consider the primitive type be to dominant. I'll put up a PR to that effect.