Nothing inferred when class type parameter is missing

[allanrenucci]

class A[T]
class B extends A

> dotc Test.scala -Xprint:frontend
result of Test.scala after frontend:

package <empty> {
  class A[T]() extends Object() { 
    T
  }
  class B() extends A[Nothing]() {}
}

Both dotty and scalac infer B extends A[Nothing]. Should we require the type parameter to be specified? Or at least emit a warning?

[smarter]

I don't think we should require the type parameter to be specified since in general we do want it to be inferrable, e.g.:

class A[T](x: T)
class B extends A(1)

Though I guess we could refuse to infer Nothing for class type parameters by default and force the user to write it explicitly.

[allanrenucci]

Are there cases where inferred Nothing or Any are not code smell?

[edmundnoble]

Inferred Nothing is fine for implicits, in a lot of cases it's necessary (for example, see https://github.com/milessabin/shapeless/blob/wip/smc/core/src/main/scala/shapeless/smc.scala#L305) but I don't think it makes sense for extends to even have type inference here.

[odersky]

I am reluctant to change it, as it might have valid uses, and the change would introduce an irregularity.