Structural types which specify concrete versions of generic methods can lead to runtime failures

[scabug]

This is extracted from the later comments of ticket #2144, which was otherwise addressed.

object Test {
  class MyGraph[V <: Any] {
    def addVertex(v: V): Boolean = true
  }

  type DuckGraph = {
    def addVertex(vertex: Int): Boolean
  }
  
  def fail(graph: DuckGraph) = graph addVertex 1

  def main(args: Array[String]): Unit = {
    fail(new MyGraph[Int])
  }
}

$$ scala Test
java.lang.NoSuchMethodException: Test$$MyGraph.addVertex(int)

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-2672?orig=1
Reporter: @paulp
See #2144

[scabug]

@dubochet said:
Upping crashers to hight priority.

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@dubochet said:
Also confirm that the fix for this bug fixes the following:

class C[+T](val x: { var y : T })
object A extends C[Int](new { var y = 1 })
(A:C[Any]).x.y = ""

java.lang.NoSuchMethodException: A$$$$anon$$1.y_$$eq(java.lang.Object)

[scabug]

Erik Bruchez (ebruchez) said:
Definitely would like to see this one fixed as it kills an ideal use case for structural types. For reference my scenario posted to scala-user:

http://goo.gl/9Y7mw

Related StackOverflow question:

http://stackoverflow.com/questions/4702664/

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@paulp said:
Assigning to meeting because I have an implementation to see.

[scabug]

@lrytz said:
Another NoSuchMethodException, maybe the same bug, maybe not:

val o = new {
  def f(x: Object) = new {
    def g(y: x.type) = 0
  }
}
val s = ""
o.f(s).g(s)

[scabug]

@paulp said:
I'd think we should just disallow that signature. It's not like it's going to know a method which takes an x.type if it sees one.

Actually that's a bug, and then there's also a bug that it's generating a call to a (String)Unit method. Where def g is declared x is an Object - I wonder how it's even getting its hands on "String"...

[scabug]

@lrytz said:
I can't follow what you are saying.

What should be disallowed? I think the code makes perfect sense, and the type checker is able to handle it correctly. Only it crashes at run-time.

scala> val o = new {
     |   def f(x: Object) = new {
     |     def g(y: x.type) = 0
     |   }
     | }
o: Object{def f(x: Object): Object{def g(y: x.type): Int}} = $anon$1@7471c619

scala> val s = ""
s: String = ""

scala> val o1 = o.f(s)
warning: there were 1 feature warnings; re-run with -feature for details
o1: Object{def g(y: s.type): Int} = $anon$1$$anon$2@2a11890d

scala> o1.g("huh")
<console>:11: error: type mismatch;
 found   : String("huh")
 required: s.type
              o1.g("huh")
                   ^

scala> o1.g(s)
warning: there were 1 feature warnings; re-run with -feature for details
java.lang.NoSuchMethodException: $anon$1$$anon$2.g(java.lang.String)
	at java.lang.Class.getMethod(Class.java:1605)
	at .reflMethod$Method1(<console>:11)
        [...]

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@paulp said:
I was thinking analogously to the restriction on abstract types, but rather than attempt to elaborate, let me just admit that I can't follow what I was saying either. I should wait for the first coffee. Strike it all from the record.

[dwijnand]

Still broken in Scala 3.0.0:

scala> object Test {
     |   class MyGraph[V <: Any] {
     |     def addVertex(v: V): Boolean = true
     |   }
     |
     |   type DuckGraph = {
     |     def addVertex(vertex: Int): Boolean
     |   }
     |
     |   def fail(graph: DuckGraph) = graph addVertex 1
     |
     |   def main(args: Array[String]): Unit = {
     |     fail(new MyGraph[Int])
     |   }
     | }
     |
10 |  def fail(graph: DuckGraph) = graph addVertex 1
   |                               ^^^^^
   |                        Found:    (graph : Test.DuckGraph)
   |                        Required: Selectable
   |
   |                        The following import might fix the problem:
   |
   |                          import reflect.Selectable.reflectiveSelectable
   |

scala> object Test {
     |   import reflect.Selectable.reflectiveSelectable
     |
     |   class MyGraph[V <: Any] {
     |     def addVertex(v: V): Boolean = true
     |   }
     |
     |   type DuckGraph = {
     |     def addVertex(vertex: Int): Boolean
     |   }
     |
     |   def fail(graph: DuckGraph) = graph addVertex 1
     |
     |   def main(args: Array[String]): Unit = {
     |     fail(new MyGraph[Int])
     |   }
     | }
// defined object Test

scala> Test.main(Array())
java.lang.NoSuchMethodException: rs$line$1$Test$MyGraph.addVertex(int)
  at java.lang.Class.getMethod(Class.java:1786)
  at scala.reflect.Selectable.applyDynamic(Selectable.scala:38)
  at scala.reflect.Selectable.applyDynamic$(Selectable.scala:11)
  at scala.reflect.Selectable$DefaultSelectable.applyDynamic(Selectable.scala:51)
  at rs$line$1$Test$.fail(rs$line$1:12)
  at rs$line$1$Test$.main(rs$line$1:15)
  ... 28 elided

Good that it requires an import, but I don't consider import reflect.Selectable.reflectiveSelectable to give that "oh this code might be unsound" feeling.

[smarter]

But not broken anymore in Scala 3 nightlies :)

15 |    fail(new MyGraph[Int])
   |         ^^^^^^^^^^^^^^^^
   |         Found:    Test.MyGraph[Int]
   |         Required: Test.DuckGraph

cf #10414 (comment)