Avoid crash if prefix if selection is higher-kinded

The test case caused a crash in NamedType, because `Lambda` is higher-kinded.
We now report an error instead.

Author: odersky

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

@@ -126,7 +126,7 @@ object ProtoTypes {
         val mbr = if (privateOK) tp1.member(name) else tp1.nonPrivateMember(name)
         def qualifies(m: SingleDenotation) =
           memberProto.isRef(defn.UnitClass) ||
-          compat.normalizedCompatible(NamedType(tp1, name, m), memberProto)
+          tp1.isValueType && compat.normalizedCompatible(NamedType(tp1, name, m), memberProto)
             // Note: can't use `m.info` here because if `m` is a method, `m.info`
             //       loses knowledge about `m`'s default arguments.
         mbr match { // hasAltWith inlined for performance

tests/neg/hkprefix.scala

@@ -0,0 +1,5 @@
+class Lambda[T] { class Tp }
+
+object Test {
+  def f(x: Lambda # Tp) = ???  // error
+}