Don't set PureInterface when a default param is present

The default param will be desugared into a method with a body, so
setting PureInterface would be wrong. The enclosed test previously
failed with a pickling difference, because the unpickler correctly
decided to not set the PureInterface flag since it saw the default param
method.

This fixes the tasty_dotc_util which failed since the last commit
because FreshNameCreator was now incorrectly recognized as a PureInterface.

Author: smarter

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

@@ -33,7 +33,11 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
   def defKind(tree: Tree): FlagSet = unsplice(tree) match {
     case EmptyTree | _: Import => NoInitsInterface
     case tree: TypeDef => if (tree.isClassDef) NoInits else NoInitsInterface
-    case tree: DefDef => if (tree.unforcedRhs == EmptyTree) NoInitsInterface else NoInits
+    case tree: DefDef =>
+      if (tree.unforcedRhs == EmptyTree)
+        (NoInitsInterface /: tree.vparamss.flatten)((fs, vparam) => fs & defKind(vparam))
+      else
+        NoInits
     case tree: ValDef => if (tree.unforcedRhs == EmptyTree) NoInitsInterface else EmptyFlags
     case _ => EmptyFlags
   }

tests/tasty/default-param-interface.scala

@@ -0,0 +1,3 @@
+trait Foo {
+  def newName(prefix: String = "foo"): String
+}