Eta expansion difference to scalac

[felixmulder]

Scalac is happy to compile the code below:

object Test {

  trait Gen[T] {
    def map[U](f: T => U): Gen[U] = ???
  }

  def f[T](implicit g: Gen[T]): Gen[() => T] =
    g map ( () => _ )
}

whereas dotty fails with:

-- Error: local/func0.scala ----------------------------------------------------
8 |    g map ( () => _ )
  |                  ^
  |    missing parameter type for parameter _$1, expected = Test.Gen[() => T]

[smarter]

dotty parser:

def f[T](implicit g: Gen[T]): Gen[() => T] =
  _$1 => g map (() => _$1)

scalac parser:

def f[T](implicit g: Gen[T]): Gen[_root_.scala.Function0[T]] =
  g.map(((x$1) => (() => x$1)))

Honestly, I would expect this to expand to () => (x => x) but apparently both parsers disagree with me ;)

[odersky]

@smarter Well spotted! I agree with scalacs parser here 😄