Class and method type parameters named _ no longer supported

[retronym]

Intentional? It is arguably poor style, but should probably be deprecated before removal.

% cat sandbox/test.scala
trait T[_] {
  def foo[_] = 0
  def bar[M[_]] = 0
}

% scalac-hash v2.11.4 ./sandbox/test.scala
warning: there was one feature warning; re-run with -feature for details
one warning found

% ./bin/dotc ./sandbox/test.scala
./sandbox/test.scala:1: error: identifier expected but '_' found.
trait T[_] {
        ^
./sandbox/test.scala:2: error: identifier expected but '_' found.
  def foo[_] = 0
          ^
two errors found

[odersky]

I would argue this should be forbidden. It's a common mistake to confuse type application and parameterized type definition. E.g. I have seen many times people write

 class Foo[Int] { ... }

and then wonder what goes wrong. Allowing an underscore for type parameter definition can only make this worse. The existing occurrences can be handled by a rewriting tool that inserts fresh identifiers for wildcards in type parameter (not type argument!) position. Assigning to Samuel.

[samuelgruetter]

I had anticipated this decision and already implemented this feature some days ago ;-)

[odersky]

Cool! Looking forward to the PR.

[smarter]

Closed because this is as intended.