Fix #1430: Avoid constrained polyparams in error message

When issuing a type mismatch error, avoid mentioning polyparams
in the current constraint set and their associated typevars. Mention
instead the bound that caused the constrained to become unsatisfiable
(if that bound is unique, i.e. the parameter appears co- or contravariantly
in the type).

Author: odersky

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

@@ -113,9 +113,23 @@ object ErrorReporting {
           case tp: TypeRef => s"with info ${tp.info} / ${tp.prefix.toString} / ${tp.prefix.dealias.toString}"
           case _ => ""
         }
+      // replace constrained polyparams and their typevars by their bounds where possible
+      val reported = new TypeMap {
+        def apply(tp: Type): Type = tp match {
+          case tp: PolyParam =>
+            val e = ctx.typerState.constraint.entry(tp)
+            if (e.exists)
+              if (variance > 0) e.bounds.hi
+              else if (variance < 0) e.bounds.lo
+              else tp
+            else tp
+          case tp: TypeVar => apply(tp.stripTypeVar)
+          case _ => mapOver(tp)
+        }
+      }
       d"""type mismatch:
            | found   : $found
-           | required: $expected""".stripMargin + whyNoMatchStr(found, expected)
+           | required: ${reported(expected)}""".stripMargin + whyNoMatchStr(found, expected)
     }
 
     /** Format `raw` implicitNotFound argument, replacing all