Passing bounded types as a wildcard

[tindzk]

Consider the following example:

case class MyList[T <: String]()
def f[T](value: T): Unit = {} 
f(null.asInstanceOf[MyList[_]])

It fails with the following error:

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

However, the above example does work when either

• explicitly passing the type: f[MyList[_]](null), or
• defining the function as def f(value: Any): Unit = {}.

I am not entirely sure whether this is the intended behaviour. In any case, it would be helpful to issue a better error message.

See also com-lihaoyi/Ammonite#629 and typelevel/scala#156.

[retronym]

There is a special case in the bounds checking for applied types that lets you just write _ rather than _ <: TheBoundsOfTheTypeCorrespondingParam. But the ill-bounded type propagates, e.g. through type inference to the type arg of f, the error is reported.

So I'm not sure if this is a bug or a "known limitation"

[soronpo]

FYI, compiles in dotty:
https://scastie.scala-lang.org/Y6FgOKcMRBegG0RwzDFUJQ