Stop rewriting withFilter to filter

This was dropped for Scala 2 in scala/scala#5252 for 2.12.0-RC1, as discussed
for instance in scala/bug#6455 and scala/bug#8339.

Beyond this code, also remove `MaybeFilter` attachment and attending code
introduced in fcea3d5.

A caveat is that, unexpectedly, this code was added in 2016-08-24, after
scala/scala#5252 was merged, as if the decision was reversed; but #1461 says:

> Also, I went through tests in pending/pos and reclassified as far as possible.

So I assume this just happened because the Scala 2 PR didn't remove our pending
test.

Author: Blaisorblade

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

@@ -10,19 +10,13 @@ import NameKinds.{UniqueName, EvidenceParamName, DefaultGetterName}
 import language.higherKinds
 import typer.FrontEnd
 import collection.mutable.ListBuffer
-import util.Property
 import reporting.diagnostic.messages._
 import reporting.trace
 
 object desugar {
   import untpd._
   import DesugarEnums._
 
-  /** Tags a .withFilter call generated by desugaring a for expression.
-   *  Such calls can alternatively be rewritten to use filter.
-   */
-  val MaybeFilter = new Property.Key[Unit]
-
   /** Info of a variable in a pattern: The named tree and its type */
   private type VarInfo = (NameTree, Tree)
 
@@ -992,12 +986,6 @@ 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 }
        *
@@ -1028,7 +1016,7 @@ object desugar {
         val cases = List(
           CaseDef(pat, EmptyTree, Literal(Constant(true))),
           CaseDef(Ident(nme.WILDCARD), EmptyTree, Literal(Constant(false))))
-        Apply(orFilter(Select(rhs, nme.withFilter)), makeCaseLambda(cases))
+        Apply(Select(rhs, nme.withFilter), makeCaseLambda(cases))
       }
 
       /** Is pattern `pat` irrefutable when matched against `rhs`?
@@ -1085,7 +1073,7 @@ object desugar {
           val vfrom1 = new IrrefutableGenFrom(makeTuple(allpats), rhs1)
           makeFor(mapName, flatMapName, vfrom1 :: rest1, body)
         case (gen: GenFrom) :: test :: rest =>
-          val filtered = Apply(orFilter(rhsSelect(gen, nme.withFilter)), makeLambda(gen.pat, test))
+          val filtered = Apply(rhsSelect(gen, nme.withFilter), makeLambda(gen.pat, test))
           val genFrom =
             if (isIrrefutableGenFrom(gen)) new IrrefutableGenFrom(gen.pat, filtered)
             else GenFrom(gen.pat, filtered)

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

@@ -462,11 +462,6 @@ class Typer extends Namer
       // 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.
       selectWithFallback(tryJavaSelectOnType(_)) // !!! possibly exponential bcs of qualifier retyping
-    else if (tree.name == nme.withFilter && tree.getAttachment(desugar.MaybeFilter).isDefined)
-      selectWithFallback {
-        implicit ctx =>
-          typedSelect(untpd.cpy.Select(tree)(tree.qualifier, nme.filter), pt) // !!! possibly exponential bcs of qualifier retyping
-      }
     else
       typeSelectOnTerm(ctx)
   }

tests/untried/neg/t6455.scala tests/neg/t6455.scalatests/untried/neg/t6455.scala renamed to tests/neg/t6455.scala

@@ -2,5 +2,5 @@ object O { def filter(p: Int => Boolean): O.type = this }
 
 class Test {
   // should not compile because we no longer rewrite withFilter => filter under -Xfuture
-  O.withFilter(f => true)
+  O.withFilter(f => true) // error // error
 }