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Regression: Tagged types are incorrectly refined #43502
Description
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
💻 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.
Activity
arcanis commentedon Apr 2, 2021
An ugly workaround if you don't use Prettier is to do (note the extra parentheses):
ahejlsberg commentedon Apr 5, 2021
Hmm, yes this is an effect of #43131. The fixes to
getTypeFactsare in principle correct, except that here we think thattypeofis both"string"and"object", which is impossible and therefore gets us tonever. 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.