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A Type Error occurs with using a generic that extends a branded intrinsic type #55040
Description
Bug Report
🔎 Search Terms
- generics
- branded
- intrinsic type
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics
⏯ Playground Link
Playground link with relevant code
💻 Code
/////////////////////////////////////////////////////////////////////////////////////////////
// Types
type BrandedNumber = number & {
__branded: true
}
type OperationIO<T extends number> = T extends BrandedNumber
? T
: number;
/////////////////////////////////////////////////////////////////////////////////////////////
// Function
function sub<T extends number>(
a: OperationIO<T>,
): (b: OperationIO<T>) => OperationIO<T> {
return (b) => (b - a) as any;
}
/////////////////////////////////////////////////////////////////////////////////////////////
// Tests
// Make sure non-branded numbers work.
const testNumbers = sub(2)(3);
// Make sure branded numbers work.
function testWithoutGenerics(a: BrandedNumber, b: BrandedNumber) {
return sub(a)(b);
}
// Subtypes of branded numbers should work too.
function testWithGenerics<T extends BrandedNumber>(a: T, b: T) {
return sub(a)(b); // <-- Why is this not allowed?
}
// Has correct typing dispite above error.
const testResultWithGenerics = testWithGenerics(
2 as BrandedNumber & { __foo: 1},
3 as BrandedNumber & { __foo: 1}
);🙁 Actual behavior
TypeScript reports a type error on line 35.
🙂 Expected behavior
No error should be reported because as far I can tell, this line should be valid. Additionally, TypeScript can successfully deduce the type on line 39.
Activity
fatcerberus commentedon Jul 17, 2023
What is an “instinct type”?
jcalz commentedon Jul 17, 2023
From context I'd say it means "primitive type"? 🤷♂️
The issue here is probably just that generic conditional types don't consult the generic's constraints and therefore remain unresolved, as in #31096
RebeccaStevens commentedon Jul 17, 2023
Yeah, it does seem to be that issue that's occurring. Any suggestions on how I can work around it? The only solution I've come up with is to use overloads of the
subfunction.Also, "instinct" was a mistake by me, I meant "intrinsic".
[-]A Type Error occurs with using a generic that extends a branded instinct type[/-][+]A Type Error occurs with using a generic that extends a branded intrinsic type[/+]jcalz commentedon Jul 17, 2023
The conventional term is "primitive", though. In TypeScript, "intrinsic" refers to compiler-provided types that users cannot implement themselves (see #40580), and while one could make a case that all primitive types are intrinsic, not all intrinsic types are primitive.
RebeccaStevens commentedon Jul 17, 2023
Ahh, good to know. I've kind of just been using the two interchangeably.
RyanCavanaugh commentedon Jul 18, 2023
For this substitution to be safe, the conditional type would have to be nondistributive. If you change this to
then it indeed works as expected