KUHRT

I was surprised by something in TypeScript today regarding the combination of:

Type Conditional
Type Parameter
Never Type

Here are some basic sub/super type tests with never:

type is_1_extends_never = 1 extends never ? true : false
// false
type is_never_extends_1 = never extends 1 ? true : false
// false
type is_never_extends_never = never extends never ? true : false
// true

No surprise (to me).

But something happens when a type parameter is introduced which I did not expect.

namespace check_is_never_extends_parameter {
	type $<p> = never extends p ? true : false

	type is_never_extends = $<1> //           never extends 1 ? true : false
	// true

	type is_never_extends_never = $<never> // never extends never ? true : false
	// true
}

namespace check_is_parameter_extends_never {
	type $<p> = p extends never ? true : false

	type is_never_extends_never = $<never> // never extends never ? true : false
	// never <------------------------------------------------------------------ ???

	type is_1_extends_never = $<1> //         1 extends never ? true : false
	// false
}

namespace check_is_1_extends_parameter {
	type $<p> = 1 extends p ? true : false

	type is_1_extends_never = $<never> // 1 extends never ? true : false
	// false

	type is_1_extends_1 = $<1> //         1 extends 1 ? true : false
	// true
}

namespace check_is_parameter_extends_1 {
	type $<p> = p extends 1 ? true : false

	type is_1_extends_never = $<never> // 1 extends never ? true : false
	// never <---------------------------------------------------------- ???

	type is_1_extends_1 = $<1> //         1 extends 1 ? true : false
	// true
}

The difference is that:

never given as a type argument
… to a conditional type
… where the respective parameter is referenced in the sub-type-position of the conditional (x (<-- sub-type position) extends y (<-- super-type position))
… evaluates to never
… instead of the conditional being evaluated (in this case, true/false).

I find this behavior counterintuitive.

Perhaps there is a good reason for it, but to me right now it feels like a bug.

I am using TypeScript version 5.8.3.

Here is a TypeScript Playground.

I filed a bug issue with TypeScript fwiw (I suspect I’ll be given an explanatioin why this is not a bug).

Observing TypeScript Never With Conditional With Parameter