Fix #2151: don't die for wrong number of typeargs applied

Author: felixmulder

compiler/src/dotty/tools/dotc/core/Decorators.scala

@@ -103,7 +103,7 @@ object Decorators {
      *  as long as `xs`.
      */
     def zipWithConserve[U](ys: List[U])(f: (T, U) => T): List[T] =
-      if (xs.isEmpty) xs
+      if (xs.isEmpty || ys.isEmpty) Nil
       else {
         val x1 = f(xs.head, ys.head)
         val xs1 = xs.tail.zipWithConserve(ys.tail)(f)

tests/neg/i2151.scala

@@ -0,0 +1,6 @@
+trait Test {
+  type Nil = [K] => K
+  type StrangeCons[H, Tail <: [H, A] => H] = Tail[H, H]
+
+  def list: StrangeCons[Int, Nil] // error
+}