Overly-narrowed generic object loses typing #57804

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1. jcalz commented on Mar 16, 2024

   

   Joe Calzaretta (jcalz)

   on Mar 16, 2024

   Last edited by jcalz

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   Your bottom examples "work" because CommonUtilFns<Common> and CommonUtilFns<A | B> is not the same as CommonUtilFns<A> | CommonUtilFns<B>. You can't just distribute across unions. CommonUtilFns<T> is invariant in T:

   declare let f: CommonUtilFns<A> | CommonUtilFns<B>
   declare let g: CommonUtilFns<A | B>
   f = g; // error!
   g = f; // error!

   Your top example doesn't work because it's essentially #30581. TS does not keep track of the identity of util here, just its type. If you have util1 and util2 both of type CommonUtilFns<A> | CommonUtilFns<B>, then if (util1.isAorB(val)) return util2.getProp(val) wouldn't be safe, right? That's the problem TS has. And that's been reported in #30581 and the recommended approach is detailed at #47109. That looks like the following:

   interface UtilMap {
   	A: A;
   	B: B;
   }
   
   const utils: { [K in keyof UtilMap]: CommonUtilFns<UtilMap[K]> } = {
   	A: {
   		isAorB: (version: Common): version is A => !!(version as A).propA,
   		getProp: (version: A) => ({ prop: version.propA }),
   	},
   	B: {
   		isAorB: (version: Common): version is B => !!(version as B).propB,
   		getProp: (version: B) => ({ prop: version.propB }),
   	}
   };
   
   function apply<K extends keyof UtilMap>(util: (typeof utils)[K], val: Common) {
   	return util.isAorB(val) ? util.getProp(val) : undefined
   }
   
   {
   	const getProp = (val: Common) => {
   		for (const util of Object.values(utils)) {
   			const ret = apply(util, val);
   			if (ret) return ret;
   		}
   		return null;
   	};
   }

   Which, yes, is a lot of weird generic stuff. See #47109. Anyway I'd say this is essentially a duplicate of #30581.

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2. nmussy commented on Mar 17, 2024

   

   Jimmy Gaussen (nmussy)

   on Mar 17, 2024

   Last edited by nmussy

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   Thanks for the detailed explanation Joe Calzaretta (@jcalz)! Your apply function was also more or less what I got recommended by Copilot, which is to do both the guarding and the prop extraction in a single function, with the same generic context. It's an understandable limitation, but a little frustrating because TS gets so close to getting it right.

   I'm going to close this issue. Maintainers, feel free to mark it as a duplicate. And thanks again for your time Joe 👍
3. Jimmy Gaussen (nmussy)

   closed this as not plannedon Mar 17, 2024
4. microsoft

   locked as resolved and limited conversation to collaborators on Oct 22, 2025