Handle implicits with default parameters.

Fixes #576.

run/pending/Meter now compiles; crashes at runtime.

Review by @smarter.

Author: odersky

src/dotty/tools/dotc/typer/Typer.scala

@@ -1447,7 +1447,13 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
               case ambi: AmbiguousImplicits =>
                 implicitArgError(s"ambiguous implicits: ${ambi.explanation} of $where")
               case failure: SearchFailure =>
-                implicitArgError(d"no implicit argument of type $formal found for $where" + failure.postscript)
+                def noImplicitFound =
+                  implicitArgError(d"no implicit argument of type $formal found for $where" + failure.postscript)
+                if (tree.symbol.hasDefaultParams) // try to append () to application
+                  tryEither
+                    { implicit ctx => typed(untpd.Apply(untpd.TypedSplice(tree), Nil), pt) }
+                    { (_, _) => noImplicitFound }
+                else noImplicitFound
             }
           }
           if (args.exists(_.isEmpty)) {

tests/pending/pos/i618.scala

@@ -0,0 +1,3 @@
+class C(val f: Any*)
+
+class D(override val f: Nothing) extends C(f)

tests/pos/i576.scala

@@ -0,0 +1,9 @@
+class A
+
+object Impl {
+  def foo()(implicit x: A = null): Int = 2
+  def test: Int = {
+    foo()() // ok
+    foo()   // did not work before, does now
+  }
+}