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.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 934ca556d..c9a2e4b48 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/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") {