Fix #5044: guard against forward references in the TypeTree of New trees

[Jasper-M]

The extra dealiasing step also avoids this scalac bug.

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[Jasper-M]

Should the following compile?

scala> def foo = { type T = Foo; val a = new T; class Foo }
def foo: Unit

Scalac doesn't think so.

[allanrenucci]

Is dealias required?

[Jasper-M]

Without it the second test still fails (crashes). If you would dealias a bit sooner I think def foo = { type T = Foo; val a = new T; class Foo } would also emit an error, like in scalac.

[allanrenucci]

I would merge the two tests together:

class I0 {
  class I1
  def test0 = {
    val x = new y.I1 // error: `y` is a forward reference extending over the definition of `x`
    val y = new I0
  }

  def test1 = {
    type T = y.I1
    val x = new T // error: `y` is a forward reference extending over the definition of `x`
    val y = new I0
  }
}

[allanrenucci]

Here is another test with a type lambda

class I0 {
  class I2[T1, T2]
  def test2 = {
    type A[T] = i5.I2[T, String]
    val i4 = new A[Int]
    val i5 = new I0
  }
}

This one currently crashes with Scala 2.12.6 😄

[allanrenucci]

I would also add:

  def test3 = {
    val x = new T // error: `T` is a forward reference extending over the definition of `x`
    val y = new I0
    type T = y.I1
  }

[allanrenucci]

LGTM. Thanks @Jasper-M