A Type Error occurs with using a generic that extends a branded intrinsic type

[RebeccaStevens]

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.

[fatcerberus]

What is an “instinct type”?

[jcalz]

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]

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 sub function.

Also, "instinct" was a mistake by me, I meant "intrinsic".

[jcalz]

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]

Ahh, good to know. I've kind of just been using the two interchangeably.

[RyanCavanaugh]

For this substitution to be safe, the conditional type would have to be nondistributive. If you change this to

type OperationIO<T extends number> = [T] extends [BrandedNumber]
 ? T
 : number;

then it indeed works as expected