Refinements to auto tupling

There's a nasty interaction with auto-tupling and trying to insert an implicit
on the qualifier of a call. If the original call fails, we need to "undo" any
auto-tupling decisions in calls where an implicit is inserted on the qualifier.

Also: Needed to fix canAutoTuple test so that Scala2 feature is checked instead of dotty's.

Also: Drop features in dotty.language that duplicate those in scala.language.

1 files changed, 21 insertions, 11 deletions

diff --git a/src/dotty/tools/dotc/typer/Applications.scala b/src/dotty/tools/dotc/typer/Applications.scala
index c8f41b7fa..efd12cb5e 100644
--- a/src/dotty/tools/dotc/typer/Applications.scala
+++ b/src/dotty/tools/dotc/typer/Applications.scala
@@ -541,17 +541,29 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
 
   def typedApply(tree: untpd.Apply, pt: Type)(implicit ctx: Context): Tree = {
 
+    /** Try same application with an implicit inserted around the qualifier of the function
+     *  part. Return an optional value to indicate success.
+     */
+    def tryWithImplicitOnQualifier(fun1: Tree, proto: FunProto)(implicit ctx: Context): Option[Tree] =
+      tryInsertImplicitOnQualifier(fun1, proto) flatMap { fun2 =>
+        tryEither { implicit ctx =>
+          Some(typedApply(
+            cpy.Apply(tree)(untpd.TypedSplice(fun2), proto.typedArgs map untpd.TypedSplice),
+            pt)): Option[Tree]
+        } { (_, _) => None }
+     }
+
     def realApply(implicit ctx: Context): Tree = track("realApply") {
-      var proto = new FunProto(tree.args, IgnoredProto(pt), this)(argCtx(tree))
-      val fun1 = typedExpr(tree.fun, proto)
+      val originalProto = new FunProto(tree.args, IgnoredProto(pt), this)(argCtx(tree))
+      val fun1 = typedExpr(tree.fun, originalProto)
 
-      // Warning: The following line is dirty and fragile. We record that auto-tupling was demanded as
+      // Warning: The following lines are dirty and fragile. We record that auto-tupling was demanded as
       // a side effect in adapt. If it was, we assume the tupled proto-type in the rest of the application.
       // This crucially relies on he fact that `proto` is used only in a single call of `adapt`,
       // otherwise we would get possible cross-talk between different `adapt` calls using the same
       // prototype. A cleaner alternative would be to return a modified prototype from `adapt` together with
       // a modified tree but this would be more convoluted and less efficient.
-      if (proto.isTupled) proto = proto.tupled
+      val proto = if (originalProto.isTupled) originalProto.tupled else originalProto
 
       // If some of the application's arguments are function literals without explicitly declared
       // parameter types, relate the normalized result type of the application with the
@@ -580,13 +592,11 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
               val result = app.result
               convertNewGenericArray(ConstFold(result))
             } { (failedVal, failedState) =>
-              val fun2 = tryInsertImplicitOnQualifier(fun1, proto)
-              if (fun1 eq fun2) {
-                failedState.commit()
-                failedVal
-              } else typedApply(
-                cpy.Apply(tree)(untpd.TypedSplice(fun2), proto.typedArgs map untpd.TypedSplice), pt)
-            }
+              def fail = { failedState.commit(); failedVal }
+              tryWithImplicitOnQualifier(fun1, originalProto).getOrElse(
+                if (proto eq originalProto) fail
+                else tryWithImplicitOnQualifier(fun1, proto).getOrElse(fail))
+             }
           case _ =>
             handleUnexpectedFunType(tree, fun1)
         }