Fix #2795: Fix is test on Names

The previous logic would have UniqueNames shadow everything else. So if one starts
with a UniqueName(prefix, sep, number), and then does a qualified name, one gets a
unique name or a qualified name. the resulting name should still be recognized as a
qualified name, however. The change in this commit achieves that.

Author: odersky

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

@@ -42,10 +42,11 @@ object NameKinds {
       override def toString = infoString
     }
 
-    /** Does this kind define logically a new name? Tested by the `rewrite` and `collect`
-     *  combinators of names.
+    /** Does this kind define logically a new name (respectively qualified name)?
+     *  Tested by the `rewrite` and `collect` combinators of class `Name`.
      */
     def definesNewName = false
+    def definesQualifiedName = false
 
     /** Unmangle simple name `name` into a name of this kind, or return
      *  original name if this is not possible.
@@ -142,6 +143,7 @@ object NameKinds {
     }
 
     override def definesNewName = true
+    override def definesQualifiedName = true
 
     def mkString(underlying: TermName, info: ThisInfo) =
       s"$underlying$separator${info.name}"

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

@@ -233,17 +233,25 @@ object Names {
       }
     }
 
+    /** Is it impossible that trees of kind `kind` qualify as
+     *  derived trees of kind `shadowed`?
+     */
+    private def shadows(kind: NameKind, shadowed: NameKind): Boolean =
+      kind.tag < shadowed.tag ||
+      kind.definesQualifiedName ||
+      kind.definesNewName && !shadowed.definesQualifiedName
+
     override def exclude(kind: NameKind): TermName = {
       val thisKind = this.info.kind
-      if (thisKind.tag < kind.tag || thisKind.definesNewName) this
+      if (shadows(thisKind, kind)) this
       else if (thisKind.tag > kind.tag) rewrap(underlying.exclude(kind))
       else underlying
     }
 
     override def is(kind: NameKind): Boolean = {
       val thisKind = this.info.kind
       thisKind == kind ||
-      !thisKind.definesNewName && thisKind.tag > kind.tag && underlying.is(kind)
+      !shadows(thisKind, kind) && underlying.is(kind)
     }
 
     @sharable // because it's just a cache for performance