Failure to lift a `FunctionN` to `Function1[TupleN[...], ...]`

[mpilquist]

Compiler version

3.0.0-RC2

Minimized code

scala> Option((1, 2, 3)).map(_ + _ + _)
val res0: Option[Int] = Some(6)

scala> def foo[T <: Tuple : Tuple.IsMappedBy[Option]](t: T)(f: Tuple.InverseMap[T, Option] => Int) = null
def foo
  [T <: Tuple]
    (t: T)
      (f: Tuple.InverseMap[T, Option] => Int)
        (implicit evidence$1: Tuple.IsMappedBy[Option][T]): Null

scala> foo(Option(1), Option(2), Option(3))(_ + _ + _)
1 |foo(Option(1), Option(2), Option(3))(_ + _ + _)
  |                                     ^^^^^^^^^
  |                                   Wrong number of parameters, expected: 1

Expectation

_ + _ + _ should be automatically lifted from Function3[Int, Int, Int, Int] to Function1[Tuple3[Int, Int, Int], Int]

[odersky]

Why "should be"? Is it a bug or a language enhancement?

[mpilquist]

@odersky Not sure which, though why does it work sometimes, like in the Option(1, 2, 3).map(_ + _ + _) example?

[odersky]

That's because of parameter untupling. Let me see whether we can generalize that.