SAM types don't support type bounds

[Arthurm1]

The following compiles in Scala but fails in Dotty (using Scastie)

object Foo {
  val map = new java.util.HashMap[String, String]
  map.computeIfAbsent("hello", foo => "world")
}

-- Error: /tmp/scastie5756006742801572629/src/main/scala/main.scala:5:31 -------
5 |  map.computeIfAbsent("hello", foo => "world")
  |                               ^^^
  |                    missing parameter type for parameter foo, expected = ?
-- [E007] Type Mismatch Error: /tmp/scastie5756006742801572629/src/main/scala/main.scala:5:38 
5 |  map.computeIfAbsent("hello", foo => "world")
  |                                      ^^^^^^^
  |         found:    String("world")
  |         required: java.util.function.Function[_ >: String, _ <: String]#R         

trying

object Foo {
  val map = new java.util.HashMap[String, String]
  map.computeIfAbsent("hello", (foo: String) => "world")
}

still results in the second error

-- [E007] Type Mismatch Error: /tmp/scastie5756006742801572629/src/main/scala/main.scala:5:48 
5 |  map.computeIfAbsent("hello", (foo: String) => "world")
  |                                                ^^^^^^^
  |         found:    String("world")
  |         required: java.util.function.Function[_ >: String, _ <: String]#R
  |         

[smarter]

@adriaanm Does scalac do something special in its handling of SAMs to deal with types like Function<? super K,? extends V> coming from Java?

[adriaanm]

No, but see scala/scala-dev#184, scala/bug#9756 and https://yanniss.github.io/varj-ecoop12.pdf

[adriaanm]

We may have accumulated some more special-casing for existentials and type inference over the years, but I can't think of us doing anything specific to SAMs. This type checks too: def foo: T forSome {type T <: String} = "a"

[smarter]

But interestingly, this doesn't typecheck in either scalac or dotty:

scala> def foo: ({type T <: String})#T = "a"
<console>:11: error: type mismatch;
 found   : String("a")
 required: AnyRef{type T <: String}#T
       def foo: ({type T <: String})#T = "a"
                                         ^

[adriaanm]

Yeah, I think it's because the existential is extrapolated to its upper bound because it appears in a covariant position.