using nominal type and structural type in self type annotation https://twitter.com/everpeace/status/973449957347753984

gistfile1.txt

 $ scala
Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_152).
Type in expressions for evaluation. Or try :help.

scala> trait Foo {}
defined trait Foo

# actually I wanted to write like this.  But it doesn't compile
# trait Bar {
#   self: { val someInt: Int } with Foo =>
# }
scala> trait Bar {
     |   self: ({
     |     type λ = { val someInt: Int }
     |     type μ = λ with Foo
     |   })#μ =>
     | }
defined trait Bar

scala> class FooBar extends Bar with Foo {
     |   val someInt = 3
     | }
defined class FooBar

Another two solutions:

From xuwei_k: https://twitter.com/xuwei_k/status/973452829326127104

 trait Bar{ self: Foo{ val someInt: Int } => }

From eed3si9n_ja: https://twitter.com/eed3si9n_ja/status/973454124585926656

object Util { type SomeInt = { val someInt: Int } }
trait Bar { self: Util.SomeInt with Foo => }