Fixed regression in lub calculation.

Changing NullaryMethodType to be a SimpleTypeProxy because nearly all
its operations forward to its result type was it seems not such a good
idea, because it also meant that calling .underlying returned the result
type rather than the method type. The way this materialized was in
subtype checks of refinement types. A lub is calculated for two nullary
method types in the course of calculating a refinement, and then the
input types are checked against the calculated lub.

However in the lub refinement, the nullary method type has become a bare
typeref, and so the subtype check failed. Closes SI-5317.

This does give me confidence that all the malformed lubs one sees
logged under -Ydebug (and there are still many, especially with type
constructors) are alerting us to real bugs elsewhere in Types.

1 files changed, 12 insertions, 0 deletions

diff --git a/test/files/pos/t5317.scala b/test/files/pos/t5317.scala
new file mode 100644
index 0000000000..8c9c9d8222
--- /dev/null
+++ b/test/files/pos/t5317.scala
@@ -0,0 +1,12 @@
+object Test {
+  trait S { type T; val x: AnyRef }
+  trait A extends S { type T <: A; val x: A = null }
+  trait B extends S { type T <: B; val x: B = null }
+  
+  val a = new A{}
+  val b = new B{}
+  val y = if (true) a else b
+  
+  // lub of y should allow for this
+  println(y.x.x)
+}