Use isRealizable to identify stable prefixes

Replaces isVolatile, which is too weak (and more complicated).
Backwards compatibility with Scala2 is ensured by dropping the
requirement in Scala2 mode.
Fixes #1047, which now compiles without inifinite recursion.

1 files changed, 31 insertions, 0 deletions

diff --git a/tests/pos/i1047.scala b/tests/pos/i1047.scala
new file mode 100644
index 000000000..0f9b54135
--- /dev/null
+++ b/tests/pos/i1047.scala
@@ -0,0 +1,31 @@
+package hello
+
+object world extends App {
+  println("hello dotty!")
+
+  trait AnimalPackage {
+    type Animal <: AnimalU
+    type AnimalU = { val age: Int }
+    def newAnimal(a: AnimalU): Animal
+    def newSubAnimal[T](a: AnimalU & T): Animal & T
+  }
+  val p: AnimalPackage = new AnimalPackage { p =>
+    type Animal = AnimalU
+    override def newAnimal(a: AnimalU): Animal = a
+    override def newSubAnimal[T](a: AnimalU & T): Animal & T = a
+  }
+  val lambda: p.Animal = p.newAnimal(new { val age = 1 })
+  trait CatPackage { pc =>
+    type Cat <: p.Animal & pc.CatDelta
+    type CatDelta = { val meow: Int }
+    type CatU = p.AnimalU & pc.CatDelta
+    def newCat(c: CatU): Cat
+    def newSubCat[T](c: CatU & T): Cat & T
+  }
+  val pc: CatPackage = new CatPackage { pc =>
+    type Cat = p.Animal & pc.CatDelta
+    def newCat(c: CatU): Cat = p.newSubAnimal[pc.CatDelta](c)
+    def newSubCat[T](c: CatU & T): Cat & T = p.newSubAnimal[pc.CatDelta & T](c)
+  }
+  val felix: pc.Cat = pc.newCat(new { val age = 1; val meow = 2 })
+}