Allow for overloaded extensions in hasExtensionMethod

This does not yet fully solve the problem of overloaded extensions,
since the identified overloaded alternatives will still be ambiguous
if the first parameter list is the same.

But at least we get a notice what extension method was tried.

Author: odersky

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

@@ -11,6 +11,7 @@ import Trees.Untyped
 import Contexts._
 import Flags._
 import Symbols._
+import Denotations.Denotation
 import Types._
 import Decorators._
 import ErrorReporting._
@@ -1204,8 +1205,12 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
    *  result matching `resultType`?
    */
   def hasExtensionMethod(tp: Type, name: TermName, argType: Type, resultType: Type)(implicit ctx: Context) = {
-    val mbr = tp.memberBasedOnFlags(name, required = ExtensionMethod)
-    mbr.exists && isApplicable(tp.select(name, mbr), argType :: Nil, resultType)
+    def qualifies(mbr: Denotation) =
+      mbr.exists && isApplicable(tp.select(name, mbr), argType :: Nil, resultType)
+    tp.memberBasedOnFlags(name, required = ExtensionMethod) match {
+      case mbr: SingleDenotation => qualifies(mbr)
+      case mbr => mbr.hasAltWith(qualifies(_))
+    }
   }
 
   /** Compare owner inheritance level.