Cannot print types with wildcards

[tindzk]

If a type with parameter bounds is instantiated with a wildcard, it cannot be printed:

@ case class MyList[T <: String]() 
defined class MyList
@ null.asInstanceOf[MyList[_]] 
cmd3.sc:8: type arguments [_$1] do not conform to class MyList's type parameter bounds [T <: String]
            .print(res3, res3, "res3", _root_.scala.None)                                                                                                                                                                        
             ^                                                                                                                                                                                                                   
cmd3.sc:8: type arguments [_$1] do not conform to class MyList's type parameter bounds [T <: String]
            .print(res3, res3, "res3", _root_.scala.None)                                                                                                                                                                        
                  ^                                                                                                                                                                                                              
Compilation Failed

The same example works fine in Typelevel's REPL.

See also typelevel/scala#156.

[lihaoyi]

Huh I thought this would have been caught by now; I guess we missed a spot

[lihaoyi]

This is a problem with PPrint. Here is a minimal repro without Ammonite:

scala> case class MyList[T <: String]()
defined class MyList

scala> println(pprint.TPrint.default[MyList[_]])
<console>:14: error: type arguments [_$1] do not conform to class MyList's type parameter bounds [T <: String]
       println(pprint.TPrint.default[MyList[_]])
                                    ^

Spent some time digging into this, didn't manage to get a satisfactory answer why this is happening. The implicit-recursion makes things kinda confusing in the TPrint macro expansion, maybe we could make it not-recurse in the case of _? That might make the problem go away, or at the very least make it easier to debug

[tindzk]

If default takes a type parameter, this most likely triggers the issue I've described. Consider the following function:

def f[T](value: T): Unit = {}

Passing a MyList[_] value to f() is not allowed in regular Scala. If you change the parameter type to Any, it compiles. I am actually curious to learn about the motivation behind this Scala design decision.

[lihaoyi]

@tindzk not sure what you are referring to, but passing a MyList[_] to f in your example works just fine.

@  case class MyList[T <: String]()
defined class MyList
@ def f[T](value: T): Unit = {}
defined function f
@ f[MyList[_]](???)
scala.NotImplementedError: an implementation is missing
  scala.Predef$.$qmark$qmark$qmark(Predef.scala:284)
  ammonite.$sess.cmd2$.<init>(cmd2.sc:1)
  ammonite.$sess.cmd2$.<clinit>(cmd2.sc)

This is a bug in the pprint.TPrint module, not in Scala itself

[tindzk]

This is what I had in mind:

scala> f(null.asInstanceOf[MyList[_]])
<console>:15: error: type arguments [_$1] do not conform to class MyList's type parameter bounds [T <: String]
       f(null.asInstanceOf[MyList[_]])

[lihaoyi]

huh, interesting. Maybe raise an issue on scala/scala? That definitely should work.

EDIT: Notably, giving an explicit type param for f makes it work, so it seems type-inference is screwing up

scala> case class MyList[T <: String]()
defined class MyList

scala> null.asInstanceOf[MyList[_]]
res0: MyList[_] = null

scala> def f[T](value: T): Unit = {}
f: [T](value: T)Unit

scala> f(null.asInstanceOf[MyList[_]])
<console>:15: error: type arguments [_$1] do not conform to class MyList's type parameter bounds [T <: String]
       f(null.asInstanceOf[MyList[_]])
       ^

scala> f[MyList[_]](null.asInstanceOf[MyList[_]])

scala>