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

@@ -31,11 +31,16 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
    *   Does tree contain an initialization part when seen as a member of a class or trait?
    */
   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: ValDef => if (tree.unforcedRhs == EmptyTree) NoInitsInterface else EmptyFlags
-    case _ => EmptyFlags
+    case EmptyTree | _: Import =>
+      NoInitsInterface
+    case tree: TypeDef =>
+      if (tree.isClassDef) NoInits else NoInitsInterface
+    case tree: DefDef =>
+      if (tree.unforcedRhs == EmptyTree && tree.vparamss.forall(_.forall(_.unforcedRhs == EmptyTree))) NoInitsInterface else NoInits
+    case tree: ValDef =>
+      if (tree.unforcedRhs == EmptyTree) NoInitsInterface else EmptyFlags
+    case _ =>
+      EmptyFlags
   }
 
   def isOpAssign(tree: Tree) = unsplice(tree) match {

tests/tasty/default-param-interface.scala

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