Closes SI-5354.

The reason why the test case compiled without error is pretty  devious: When checking the `Foo.x' implicit, a CyclicReference error occurs which causes the alternative to be discarded. Why a CylicReference? Because the inferencer tries to decide whether the owner of `z` is a subclass of the owner od `x`. To do this, it computed the info of the owner of `z1`, which is not complete because no result type for `f1` was given. Hence a CyclicReference error.

The fix is twofold: (1) We make isNonBottomSubClass smarter so that it always returns false if the symbol in question is not a type; hence the info need not be computed. (2) It's dubious to swallow CyclicReference errors anywhere, but I deemed it too risky to propagate them. But at least the CyclicReference is now logged if -Ylog-implicit is true. This hopefully spares future maintainers the same detective work I had to go through when digging this out.

1 files changed, 8 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index d54cb248cf..77dde88a80 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -816,7 +816,14 @@ trait Implicits {
               val newPending = undoLog undo {
                 is filterNot (alt => alt == i || {
                   try improves(i, alt)
-                  catch { case e: CyclicReference => true }
+                  catch { 
+                    case e: CyclicReference => 
+                      if (printInfers) {
+                        println(i+" discarded because cyclic reference occurred")
+                        e.printStackTrace()
+                      }
+                      true 
+                  }
                 })
               }
               rankImplicits(newPending, i :: acc)