Handle multiple val _ = ... definitions

These need to be expanded to pattern definitions, not ValDefs,
otherwise duplicate definition errors result.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1616,7 +1616,7 @@ object desugar {
    */
   private object IdPattern {
     def unapply(tree: Tree)(implicit ctx: Context): Option[VarInfo] = tree match {
-      case id: Ident => Some(id, TypeTree())
+      case id: Ident if id.name != nme.WILDCARD => Some(id, TypeTree())
       case Typed(id: Ident, tpt) => Some((id, tpt))
       case _ => None
     }

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2574,7 +2574,7 @@ object Parsers {
           }
         } else EmptyTree
       lhs match {
-        case (id @ Ident(name: TermName)) :: Nil =>
+        case (id @ Ident(name: TermName)) :: Nil if name != nme.WILDCARD =>
           val vdef = ValDef(name, tpt, rhs)
           if (isBackquoted(id)) vdef.pushAttachment(Backquoted, ())
           finalizeDef(vdef, mods, start)

tests/pos/wildcardDefs.scala

@@ -0,0 +1,4 @@
+object Test {
+  val _ = 2
+  val _ = 3
+}