Can't expect type on a type intersection

[Ethan-Arrowood]

I'm getting this error message from my unit test:

Parameter type HeadersType is declared too wide for argument type IncomingHttpHeaders & HeadersType.

I understand this has something to do with expectType being is strict mode (#40). Can you please share how I could improve my type check here? Should I use something other than void?

The type definition (simplified)

interface CustomHeaders<G> {
  headers: http.IncomingHttpHeaders & G
}

And my test implementation (simplified)

type HeadersType = { prop: string }
const req: CustomHeaders<HeadersType> = { headers: { prop: 'Header' } }
expectType<HeadersType>(obj.headers)

I made a typescript playground that implements this pattern so I don't know why its failing in TSD

[Ethan-Arrowood]

If you want to see the whole example / where I discovered this issue checkout the reference issue ^^ from the Fastify library

[ExE-Boss]

You need to call:

expectType<http.IncomingHttpHeaders & HeadersType>(obj.headers);

[Ethan-Arrowood]

Thank you @ExE-Boss