What does the "_" type mean in swift error messages?

Question

Occasionally when using generics, I get an error message that refers to a "_" as a parameter. It doesn't appear to be documented. What does it mean?

As an example, I get the error:

Cannot convert value of type 'JawDroppingFeat<Superhero>' to closure result type 'JawDroppingFeat<_>'

when I try to compile:

protocol SuperheroType {
  typealias Superpower
}

struct JawDroppingFeat<Superhero: SuperheroType where Superhero: Arbitrary, Superhero.Superpower: Arbitrary>: Arbitrary {
  let subject: Superhero
  let superpowerUsed: Superhero.Superpower

  static var arbitrary: Gen<JawDroppingFeat<Superhero>> {
    get {
      return Gen.zip(Superhero.arbitrary, Superhero.Superpower.arbitrary)
        .map{ (x: Superhero, y: Superhero.Superpower) in
          JawDroppingFeat(subject: x, superpowerUsed: y)
        }
    }
  }
}

The Gen and Arbitrary types are from SwiftCheck and the relevant declarations are:

public struct Gen<A> {
  public static func zip<A, B>(gen1: SwiftCheck.Gen<A>, _ gen2: SwiftCheck.Gen<B>) -> SwiftCheck.Gen<(A, B)>
  public func map<B>(f: A -> B) -> SwiftCheck.Gen<B>
}

public protocol Arbitrary {
  public static var arbitrary: SwiftCheck.Gen<Self> { get }
}

I assume that the <_> is something to do with swift failing to infer the type parameter rather than an image of Chris Lattner wincing at me. But does it have a more precise (and documented) meaning?

Edit

My best current theory is that when Swift fails to infer a type parameter, instead of failing immediately, it assigns a null (_) type, which results in the actual compile error downstream at some point where an incompatible type (in my case, the parameters to .map) is passed.

Answer 1

It means you have incomplete type information in a return value of parameter.

In this case the .map function is returning a generic JawDroppingFeat for which you did not specify the embedded type.

I assume you meant to write

 JawDroppingFeat<SuperHero>(subject: x, superpowerUsed: y)