Notice when erasure is trying to cast something...

Notice when erasure is trying to cast something to Unit, and don't
do it. I'm not sure at what prior point such things should have been
caught, but for now we can have a sanity check. Closes #4731, review by
odersky.

2 files changed, 23 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/Erasure.scala b/src/compiler/scala/tools/nsc/transform/Erasure.scala
index c86a7b5867..5f1d5808b4 100644
--- a/src/compiler/scala/tools/nsc/transform/Erasure.scala
+++ b/src/compiler/scala/tools/nsc/transform/Erasure.scala
@@ -627,8 +627,14 @@ abstract class Erasure extends AddInterfaces
     /** Generate a synthetic cast operation from tree.tpe to pt.
      *  @pre pt eq pt.normalize
      */
-    private def cast(tree: Tree, pt: Type): Tree =
-      tree AS_ATTR pt
+    private def cast(tree: Tree, pt: Type): Tree = {
+      if (pt.typeSymbol == UnitClass) {
+        // See SI-4731 for one example of how this occurs.
+        log("Attempted to cast to Unit: " + tree)
+        tree.duplicate setType pt
+      }
+      else tree AS_ATTR pt
+    }
 
     private def isUnboxedValueMember(sym: Symbol) =
       sym != NoSymbol && isValueClass(sym.owner)
@@ -647,7 +653,7 @@ abstract class Erasure extends AddInterfaces
       else if (isValueClass(tree.tpe.typeSymbol) && !isValueClass(pt.typeSymbol))
         adaptToType(box(tree), pt)
       else if (tree.tpe.isInstanceOf[MethodType] && tree.tpe.params.isEmpty) {
-        if (!tree.symbol.isStable) assert(false, "adapt "+tree+":"+tree.tpe+" to "+pt)
+        assert(tree.symbol.isStable, "adapt "+tree+":"+tree.tpe+" to "+pt)
         adaptToType(Apply(tree, List()) setPos tree.pos setType tree.tpe.resultType, pt)
       } else if (pt <:< tree.tpe)
         cast(tree, pt)
diff --git a/test/files/pos/bug4731.scala b/test/files/pos/bug4731.scala
new file mode 100644
index 0000000000..d457543c1f
--- /dev/null
+++ b/test/files/pos/bug4731.scala
@@ -0,0 +1,14 @@
+import java.util.Comparator
+
+trait Trait1[T] { def foo(arg: Comparator[T]): Unit }
+
+trait Trait2[T] extends Trait1[T] { def foo(arg: Comparator[String]): Int = 0 }
+
+class Class1 extends Trait2[String] { }
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val c = new Class1
+    c.foo(Ordering[String])
+  }
+}