Fix problem when handling structural types without a nominal parent type.

We need to use Object as parent then, but this was forgotten.

Author: odersky

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

@@ -715,10 +715,12 @@ object desugar {
    *  ==>
    *      trait <refinement> extends parent { refinements }
    *
+   *  If the parent is missing, Object is assumed.
    *  The result is used for validity checking, is thrown away afterwards.
    */
   def refinedTypeToClass(tree: RefinedTypeTree)(implicit ctx: Context): TypeDef = {
-    val impl = Template(emptyConstructor, tree.tpt :: Nil, EmptyValDef, tree.refinements)
+    val parent = if (tree.tpt.isEmpty) TypeTree(defn.ObjectType) else tree.tpt
+    val impl = Template(emptyConstructor, parent :: Nil, EmptyValDef, tree.refinements)
     TypeDef(Modifiers(Trait), tpnme.REFINE_CLASS, impl)
   }
 

src/dotty/tools/dotc/typer/Typer.scala

@@ -798,7 +798,7 @@ class Typer extends Namer with Applications with Implicits {
   }
 
   def typedRefinedTypeTree(tree: untpd.RefinedTypeTree)(implicit ctx: Context): RefinedTypeTree = track("typedRefinedTypeTree") {
-    val tpt1 = typedAheadType(tree.tpt)
+    val tpt1 = if (tree.tpt.isEmpty) TypeTree(defn.ObjectType) else typedAheadType(tree.tpt)
     val refineClsDef = desugar.refinedTypeToClass(tree)
     val refineCls = createSymbol(refineClsDef).asClass
     val TypeDef(_, _, Template(_, _, _, refinements1)) = typed(refineClsDef)

test/dotc/tests.scala

@@ -39,6 +39,7 @@ class tests extends CompilerTest {
   @Test def pos_packageobject() = compileFile(posDir, "packageobject")
   @Test def pos_overloaded() = compileFile(posDir, "overloaded")
   @Test def pos_templateParents() = compileFile(posDir, "templateParents")
+  @Test def pos_structural() = compileFile(posDir, "structural")
 
   @Test def neg_blockescapes() = compileFile(negDir, "blockescapesNeg", xerrors = 1)
   @Test def neg_typedapply() = compileFile(negDir, "typedapply", xerrors = 4)