Variance annotations are not allowed in type aliases for interfaces

[bgenia]

🔎 Search Terms

variance annotations

🕗 Version & Regression Information

• This seems to be a bug
• This is the behavior in every version I tried since variance annotations came out

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAKgYgewQHgGoEMA2BXCA+KAXigG8oA3LXALigx2gF8AoUSWAIXQCdkBLAHZQE2YHSr4isRCnq4CAegVQA8gGlmzQcAjcAZugDG0AJIy0EgmUoNacploE79R0116Dho8QwIQAHjoCACYAzlBmSBa+pIxQSqoarODQ7vxCImL2BMQmadnxygCi3NwI3MxAA

💻 Code

type TFoo<Value> = { value: Value }
type TBar<in out Value> = TFoo<Value> // OK

interface IFoo<Value> { value: Value }
interface IBar<in out Value> extends IFoo<Value> {} // OK

type Bar<in out Value> = IBar<Value> // Error

🙁 Actual behavior

The error says that variance annotations are only allowed for object types, but IBar is an object type.

🙂 Expected behavior

There should be no errors

Additional information about the issue

No response

[RyanCavanaugh]

This is a correct error because it prevents a footgun with variance annotations.

So let's say you have

type TypeRef<T> = TypeTarget<T>;
type TypeTarget<T> = TypeFinal<T>;
type TypeFinal<T> = { value: T };

Given an assignability check from TypeRef<Source> to TypeRef<Target>, we could in theory do any of these three operations equivalently (since they all eventually end up at the same place):

• Source to Target (if TypeRef is covariant on T)
• TypeTarget<Source> to TypeTarget<Target>
• TypeFinal<Source> to TypeFinal<Target>
• Source to Target (via the value property)

For performance reasons, it's best to start as low in the stack as possible, so when there's an immediate type alias, we instantly go to the second bullet point and do the comparison starting there. This means any variance annotations present on TypeRef are ignored.

This is not a bug, because variance annotations are not intended to change the variance of a type. If you wrote a variance annotation that isn't the observed variance of a type, that's an operator error. In this case since we know the variance annotations on Bar aren't ever going to be used, we should be erroring since you're clearly annotating the wrong place (and probably shouldn't be annotating at all).

This is also somewhat alluded to to in the FAQ https://github.com/Microsoft/TypeScript/wiki/FAQ#structural-vs-instantiation-based-inference as regards to inference of type parameters (rather than comparison, as occurs here)

The Handbook entry really needs to emphasize that variance annotations are not for changing the variance of a type parameter, despite all outward appearances.