SI-8035 Deprecate automatic () insertion in argument lists

This promotes the () insertion warning from -Ywarn-adapted-args to a
deprecation warning. -Xfuture tunrs it into a compiler error.

Auto tupling remains unchanged for now.

The tests have been fixed the following way:
- Warnings caused by general sloppiness (Try(), Future(), ...) have been
  fixed.
- Warnings which raise interesting questions (x == (), ...) received an
  updated checkfile for now.

1 files changed, 16 insertions, 13 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Adaptations.scala b/src/compiler/scala/tools/nsc/typechecker/Adaptations.scala
index 0b5b0759b2..1e544e54f6 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Adaptations.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Adaptations.scala
@@ -43,11 +43,11 @@ trait Adaptations {
       def givenString = if (args.isEmpty) "<none>" else args.mkString(", ")
       def adaptedArgs = if (args.isEmpty) "(): Unit" else args.mkString("(", ", ", "): " + applyArg.tpe)
 
-      def adaptWarning(msg: String) = context.warning(t.pos, msg +
+      def adaptWarningMessage(msg: String, showAdaptation: Boolean = true) = msg +
         "\n        signature: " + sigString +
         "\n  given arguments: " + givenString +
-        "\n after adaptation: " + callString + "(" + adaptedArgs + ")"
-      )
+        (if (showAdaptation) "\n after adaptation: " + callString + "(" + adaptedArgs + ")" else "")
+
       // A one-argument method accepting Object (which may look like "Any"
       // at this point if the class is java defined) is a "leaky target" for
       // which we should be especially reluctant to insert () or auto-tuple.
@@ -69,17 +69,20 @@ trait Adaptations {
       }
 
       if (settings.noAdaptedArgs)
-        adaptWarning("No automatic adaptation here: use explicit parentheses.")
-      else if (settings.warnAdaptedArgs)
-        adaptWarning(
-          if (args.isEmpty) "Adapting argument list by inserting (): " + (
-            if (isLeakyTarget) "leaky (Object-receiving) target makes this especially dangerous."
-            else "this is unlikely to be what you want."
-          )
-          else "Adapting argument list by creating a " + args.size + "-tuple: this may not be what you want."
-        )
+        context.warning(t.pos, adaptWarningMessage("No automatic adaptation here: use explicit parentheses."))
+      else if (args.isEmpty) {
+        if (settings.future)
+          context.error(t.pos, adaptWarningMessage("Adaptation of argument list by inserting () has been removed.", showAdaptation = false))
+        else {
+          val msg = "Adaptation of argument list by inserting () has been deprecated: " + (
+          if (isLeakyTarget) "leaky (Object-receiving) target makes this especially dangerous."
+          else "this is unlikely to be what you want.")
+          context.unit.deprecationWarning(t.pos, adaptWarningMessage(msg))
+        }
+      } else if (settings.warnAdaptedArgs)
+        context.warning(t.pos, adaptWarningMessage(s"Adapting argument list by creating a ${args.size}-tuple: this may not be what you want."))
 
-      !settings.noAdaptedArgs
+      !settings.noAdaptedArgs || !(args.isEmpty && settings.future)
     }
   }
 }