Partially fix #1500: Implicit search breaks at a certain depth

The issue fixed here was introduced by 71027f15. The added `csyms.isEmpty` condition on `case nil =>` is always true, which is clearely a bug.

t1500c still fails with covariant (or contravariant) type parameters on `::`, but this seams to be a more complicated issue involving the typer.

1 files changed, 21 insertions, 0 deletions

diff --git a/tests/run/t1500b.scala b/tests/run/t1500b.scala
new file mode 100644
index 000000000..8b52731a5
--- /dev/null
+++ b/tests/run/t1500b.scala
@@ -0,0 +1,21 @@
+sealed trait Nat
+sealed trait Succ[Prev <: Nat] extends Nat
+sealed trait Zero extends Nat
+
+case class ToInt[N <: Nat](value: Int)
+
+object ToInt {
+  implicit val caseZero: ToInt[Zero] = ToInt(0)
+  implicit def caseSucc[Prev <: Nat](implicit e: ToInt[Prev]): ToInt[Succ[Prev]] = ToInt(e.value + 1)
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    assert(implicitly[ToInt[Zero]].value == 0)
+    assert(implicitly[ToInt[Succ[Zero]]].value == 1)
+    assert(implicitly[ToInt[Succ[Succ[Zero]]]].value == 2)
+    assert(implicitly[ToInt[Succ[Succ[Succ[Zero]]]]].value == 3)
+    assert(implicitly[ToInt[Succ[Succ[Succ[Succ[Zero]]]]]].value == 4)
+    assert(implicitly[ToInt[Succ[Succ[Succ[Succ[Succ[Zero]]]]]]].value == 5)
+  }
+}