typescript - generics - reference to parameter from same parameter list

Question

Can somebody explain why the following isn't allowed in Typescript?

interface Base<T> {
}

interface SomeInterface<First, Second extends Base<First>> {
}

For some reason I do not understand, it is not possible to refer to type First in Base<First>.

The error is: Constraint of type parameter cannot reference any type parameter from the same parameter list.

If an answer was provided, you should really not just read & ignore it, but provide feedback and, if it answered the question fully, mark it as accepted. — Ingo Bürk
– Ingo Bürk, Commented Mar 23, 2015 at 17:28

Ingo Bürk · Accepted Answer · 2015-03-22 21:08:56Z

2

According to the wiki on breaking changes for 0.9.7, this was introduced to reduce overhead:

Instead, use constraint arguments that are not in the same argument list, make constraints non-generic, or simply pass 'any' as the parameter to the constraint.

Reason: The added overhead in terms of type-checking, error-reporting, and design complexity did not add enough additional expressiveness to make it worthwhile for 1.0. We may revisit this in future versions of TypeScript.

Your usecase is exactly one of the two mentioned ones in the documentation there.

answered Mar 22, 2015 at 21:08

Ingo Bürk

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