Give an error when constraints are unsatisfiable

Not sure this is necessary, but it's not obvious that having *another* error
downstream is guaranteed, so giving one right away sounds safer.

OTOH, it's not immediate to give a *good* error (which bound is violated?), and
if there is a check downstream I'd rather rely on that.

Author: Blaisorblade

compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -480,7 +480,9 @@ object ProtoTypes {
       else tl
     val added = ensureFresh(tl)
     val tvars = if (addTypeVars) newTypeVars(added) else Nil
-    ctx.typeComparer.addToConstraint(added, tvars.tpes.asInstanceOf[List[TypeVar]])
+    if (!ctx.typeComparer.addToConstraint(added, tvars.tpes.asInstanceOf[List[TypeVar]])) {
+      ctx.error(s"Unsatisfiable type parameter constraints in polymorphic application", owningTree.pos)
+    }
     (added, tvars)
   }
 

tests/neg/i4721.scala

@@ -1,4 +1,4 @@
 object Test {
-  def main(args: Array[String]):Unit = m(1) // error
+  def main(args: Array[String]):Unit = m(1) // error // error
   def m[Y<:String, Z>:Int, W>:Z<:Y](d:Y):Unit={}
 }