Another existential problem down.

There is a window of danger when multiple related elements are
being typed where something which is conceptually one thing can
slip into two things, and those two things can be incompatible
with one another.  Less mysteriously, c478eb770d fixed this:

  def f = { object Bob ; Bob } ; val g = f

But, it did not fix this:

  def f = { case class Bob() ; Bob } ; val g = f

See test case pos/existentials-harmful.scala for an "in the wild"
code example fixed by this commit.

The root of the problem was that the getter and the field would each
independently derive the same existential type to describe Bob, but
those existentials were not the same as one another.

This has been the most elusive bug I have ever fixed. I want to cry when
I think of how much time I've put into it over the past half decade or
so. Unfortunately the way the repl works it is particularly good at
eliciting those grotesque found/required error messages and so I was
never able to let the thing go.

There is still a cosmetic issue (from the last commit really) where
compound types wind up with repeated parents.

Closes SI-1195, SI-1201.

1 files changed, 73 insertions, 0 deletions

diff --git a/test/files/run/existentials3.scala b/test/files/run/existentials3.scala
new file mode 100644
index 0000000000..bb80d366cc
--- /dev/null
+++ b/test/files/run/existentials3.scala
@@ -0,0 +1,73 @@
+object Test {
+  trait ToS { final override def toString = getClass.getName }
+
+  def f1 = { case class Bar() extends ToS; Bar }
+  def f2 = { case class Bar() extends ToS; Bar() }
+  def f3 = { class Bar() extends ToS; object Bar extends ToS; Bar }
+  def f4 = { class Bar() extends ToS; new Bar() }
+  def f5 = { object Bar extends ToS; Bar }
+  def f6 = { () => { object Bar extends ToS ; Bar } }
+  def f7 = { val f = { () => { object Bar extends ToS ; Bar } } ; f }
+
+  def f8 = { trait A ; trait B extends A ; class C extends B with ToS; new C { } }
+  def f9 = { trait A ; trait B ; class C extends B with A with ToS; new C { } }
+  
+  def f10 = { class A { type T1 } ; List[A#T1]() }
+  def f11 = { abstract class A extends Seq[Int] ; List[A]() }
+  def f12 = { abstract class A extends Seq[U forSome { type U <: Int }] ; List[A]() }
+  
+  val g1 = { case class Bar() extends ToS; Bar }
+  val g2 = { case class Bar() extends ToS; Bar() }
+  val g3 = { class Bar() extends ToS; object Bar extends ToS; Bar }
+  val g4 = { class Bar() extends ToS; new Bar() }
+  val g5 = { object Bar extends ToS; Bar }
+  val g6 = { () => { object Bar extends ToS ; Bar } }
+  val g7 = { val f = { () => { object Bar extends ToS ; Bar } } ; f }
+
+  val g8 = { trait A ; trait B extends A ; class C extends B with ToS; new C { } }
+  val g9 = { trait A ; trait B ; class C extends B with A with ToS; new C { } }
+
+  val g10 = { class A { type T1 } ; List[A#T1]() }
+  val g11 = { abstract class A extends Seq[Int] ; List[A]() }
+  val g12 = { abstract class A extends Seq[U forSome { type U <: Int }] ; List[A]() }
+  
+  def m[T: Manifest](x: T) = println(manifest[T])
+  
+  // manifests don't work for f10/g10
+  def main(args: Array[String]): Unit = {
+    m(f1)
+    m(f2)
+    m(f3)
+    m(f4)
+    m(f5)
+    m(f6)
+    m(f7)
+    m(f8)
+    m(f9)
+    // m(f10)
+    m(f11)
+    m(f12)
+    m(g1)
+    m(g2)
+    m(g3)
+    m(g4)
+    m(g5)
+    m(g6)
+    m(g7)
+    m(g8)
+    m(g9)
+    // m(g10)
+    m(g11)
+    m(g12)
+  }
+}
+
+object Misc {
+  trait Bippy { def bippy = "I'm Bippy!" }
+  object o1 {
+    def f1 = { trait A extends Seq[U forSome { type U <: Bippy }] ; abstract class B extends A ; trait C extends B ; (null: C) }
+    def f2 = f1.head.bippy
+  }
+  def g1 = o1.f1 _
+  def g2 = o1.f2 _
+}