[swift-users] Filling two type parameters with the same type

[David Sweeris]

Hmm... I just tried it in a playground, and it works:
struct DistortedNoise<Source, Displacement> {
  let source:Source,
  displacement:Displacement
  
  init(source:Source, displacement:Displacement)
  {
    self.source       = source
    self.displacement = displacement
  }
  
}
extension DistortedNoise where Source == Displacement {
  init(source:Source)
  {
    self.source       = source
    self.displacement = source
  }
}
let foo = DistortedNoise(source: 1) // DistortedNoise<Int, Int>
foo.source // 1
foo.displacement // 1

That's with Xcode 9b2, using the default Xcode 9.0 toolchain. With Xcode 8.3.3 and its default toolchain, the `let foo = ...` line outputs "__lldb_expr_2.DistortedNoise<Int, Int>" instead of just "DistortedNoise<Int, Int>", but that's just a more... I think the word is "qualified"... name for the same thing.

- Dave Sweeris


> On Jul 8, 2017, at 9:39 PM, Zhao Xin <owenzx at gmail.com> wrote:
> 
> No, David, it is now allowed.  "error: same-type requirement makes generic parameters 'Source' and 'Displacement' equivalent".
> 
> Zhao Xin
> 
> On Sun, Jul 9, 2017 at 12:35 PM, David Sweeris via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> You could try putting that init in an extension "where Source == Displacement"
> 
> > On Jul 8, 2017, at 21:06, Taylor Swift via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> >
> > I have a type like
> >
> > struct DistortedNoise<Source, Displacement> where Source:Noise, Displacement:Noise
> > {
> >     let source:Source,
> >         displacement:Displacement
> >
> >     init(source:Source, displacement:Displacement)
> >     {
> >         self.source       = source
> >         self.displacement = displacement
> >     }
> >
> >     init(source:Source)
> >     {
> >         self.source       = source
> >         self.displacement = source
> >     }
> > }
> >
> > and I get the error
> >
> > Compile Swift Module 'Noise' (5 sources)
> > /home/taylor/noise/sources/noise/noise.swift:576:29: error: 'Source' is not convertible to 'Displacement'
> >         self.displacement = source
> >                             ^~~~~~
> >
> > How do I tell Swift that I want the same type fulfilling both Source and Displacement?
> >
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