Fix #2473: Check self for inner class redefinitions

[nicolasstucki]

No description provided.

[smarter]

But it's not an override, it's a shadowing, right?

[nicolasstucki]

Oh, yes. I reused the old error message. I will improve it to make the distinction.

[smarter]

In fact, I don't think that shadowing is the right term either, consider:

trait Foo {
  val x: Int = 1
}

trait Bar { foo: Foo =>
  val x: String = ""
}

In both scalac and dotty, this code compiles, but if you try to make an instance of Bar, it fails:

object Test {
  def main(args: Array[String]): Unit = {
    new Foo with Bar {}
  }
}

-- Error: try/i2473.scala:14:4 -------------------------------------------------
14 |    new Foo with Bar {}
   |    ^
   |   class $anon inherits conflicting members:
   |     value x in trait Foo of type Int  and
   |     value x in trait Bar of type String
   |   (Note: this can be resolved by declaring an override in class $anon.)

[nicolasstucki]

I changed the message to print

-- [E068] Syntax Error: tests/neg/i2473.scala:3:7 ------------------------------
3 |  type Inner <: foo.Inner // error
  |       ^
  |type Inner cannot have the same name as trait Inner in trait Foo -- cannot define type member with the same name as a trait member in self reference foo.
  |(Note: this can be resolved by using another name)
-- [E068] Syntax Error: tests/neg/i2473.scala:6:8 ------------------------------
6 |  class Inner // error
  |        ^
  |class Inner cannot have the same name as trait Inner in trait Foo -- cannot define class member with the same name as a trait member in self reference baz.
  |(Note: this can be resolved by using another name)

[smarter]

typo: reson -> reason