Fix problem in TermRef.alternatives

Rewrap needs to produce alternatives with signatures. Otgerwise the new denotation will simply overwrite the old because both the overloaded TermRef and the alternative will hash to the same unique TermRef.

Author: odersky

src/dotty/tools/dotc/core/Types.scala

@@ -1140,7 +1140,7 @@ object Types {
     override def isOverloaded(implicit ctx: Context) = denot.isOverloaded
 
     private def rewrap(sd: SingleDenotation)(implicit ctx: Context) =
-      TermRef(prefix, name, sd)
+      TermRef.withSig(prefix, name, sd.signature, sd)
 
     def alternatives(implicit ctx: Context): List[TermRef] =
       denot.alternatives map rewrap