Implement alternative desugaring of for-if to filter.

Fallback to .filter if a .withFilter is not available, but do this
only for .withFilter calls generated from for expressions (this is
different from what scalac does; the latter can also rewrite
.withFilter calls given in source, which is not desirable.

Author: odersky

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

@@ -8,6 +8,7 @@ import SymDenotations._, Symbols._, StdNames._, Annotations._, Trees._
 import Decorators._
 import language.higherKinds
 import collection.mutable.ListBuffer
+import util.Attachment
 import config.Printers._
 
 object desugar {
@@ -17,6 +18,11 @@ object desugar {
 
   import untpd._
 
+  /** Tags a .withFilter call generated by desugaring a for expression.
+   *  Such calls can alternatively be rewritten to use filter.
+   */
+  val MaybeFilter = new Attachment.Key[Unit]
+
   /** Info of a variable in a pattern: The named tree and its type */
   private type VarInfo = (NameTree, Tree)
 
@@ -773,6 +779,12 @@ object desugar {
           (Bind(name, pat), Ident(name))
       }
 
+      /** Add MaybeFilter attachment */
+      def orFilter(tree: Tree): tree.type = {
+        tree.putAttachment(MaybeFilter, ())
+        tree
+      }
+
       /** Make a pattern filter:
        *    rhs.withFilter { case pat => true case _ => false }
        *
@@ -803,7 +815,7 @@ object desugar {
         val cases = List(
           CaseDef(pat, EmptyTree, Literal(Constant(true))),
           CaseDef(Ident(nme.WILDCARD), EmptyTree, Literal(Constant(false))))
-        Apply(Select(rhs, nme.withFilter), makeCaseLambda(cases))
+        Apply(orFilter(Select(rhs, nme.withFilter)), makeCaseLambda(cases))
       }
 
       /** Is pattern `pat` irrefutable when matched against `rhs`?
@@ -858,7 +870,7 @@ object desugar {
           val vfrom1 = new IrrefutableGenFrom(makeTuple(allpats), rhs1)
           makeFor(mapName, flatMapName, vfrom1 :: rest1, body)
         case (gen: GenFrom) :: test :: rest =>
-          val filtered = Apply(rhsSelect(gen, nme.withFilter), makeLambda(gen.pat, test))
+          val filtered = Apply(orFilter(rhsSelect(gen, nme.withFilter)), makeLambda(gen.pat, test))
           val genFrom =
             if (isIrrefutableGenFrom(gen)) new IrrefutableGenFrom(gen.pat, filtered)
             else GenFrom(gen.pat, filtered)

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

@@ -346,11 +346,17 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       }
     }
 
-    if (ctx.compilationUnit.isJava && tree.name.isTypeName) {
+    def selectWithFallback(fallBack: => Tree) =
+      tryEither(tryCtx => asSelect(tryCtx))((_, _) => fallBack)
+
+    if (ctx.compilationUnit.isJava && tree.name.isTypeName)
       // SI-3120 Java uses the same syntax, A.B, to express selection from the
       // value A and from the type A. We have to try both.
-      tryEither(tryCtx => asSelect(tryCtx))((_, _) => asJavaSelectFromTypeTree(ctx))
-    } else asSelect(ctx)
+      selectWithFallback(asJavaSelectFromTypeTree(ctx))
+    else if (tree.name == nme.withFilter && tree.getAttachment(desugar.MaybeFilter).isDefined)
+      selectWithFallback(typedSelect(untpd.cpy.Select(tree)(tree.qualifier, nme.filter), pt))
+    else
+      asSelect(ctx)
   }
 
   def typedSelectFromTypeTree(tree: untpd.SelectFromTypeTree, pt: Type)(implicit ctx: Context): Tree = track("typedSelectFromTypeTree") {

tests/pending/pos/t7035.scala renamed to tests/disabled/not-representable/t7035.scala

@@ -1,3 +1,5 @@
+// no longer works because dotty uses name-nased pattern matching for case classes
+
 case class Y(final var x: Int, final private var y: String, final val z1: Boolean, final private val z2: Any) {
 
   import Test.{y => someY}

tests/pending/pos/t7228.scala renamed to tests/disabled/not-representable/t7228.scala

@@ -1,3 +1,4 @@
+// no longer works because dotty does not have a concept of weak conformance
 object AdaptWithWeaklyConformantType {
   implicit class D(d: Double) { def double = d*2 }
 

tests/pos/for-filter.scala

@@ -0,0 +1,12 @@
+object Test {
+
+  case class C[T](xs: List[T]) {
+    def filter(p: T => Boolean) = new C(xs.filter(p))
+    def map[U](f: T => U) = new C(xs.map(f))
+  }
+
+  def main(args: Array[String]): Unit =
+    println(for (x <- C(List(1, 2, 3)) if x % 2 == 0) yield x)
+    // println(C(List(1, 2, 3)).withFilter(_ % 2 == 0))    // error
+
+}