SI-7294 Treat TupleN as final under -Xfuture

For the purposes of checkability warnings. This will
warn in case of:

	scala> (1, 2) match { case Seq() => 0; case _ => 1 }
	res9: Int = 1

Given how often Tuples are used as scrutinees, this is
a highly desirable place to warn.

I was orginally going to unlock this under -Xlint, and
could be easily convinced to go that way, given that
-Xfuture is a less popular option.

5 files changed, 27 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Checkable.scala b/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
index 88bfa6099d..026f5f7bc8 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
@@ -195,11 +195,18 @@ trait Checkable {
      *  so I will consult with moors about the optimal time to be doing this.
      */
     def areIrreconcilableAsParents(sym1: Symbol, sym2: Symbol): Boolean = areUnrelatedClasses(sym1, sym2) && (
-         sym1.initialize.isEffectivelyFinal // initialization important
-      || sym2.initialize.isEffectivelyFinal
+         isEffectivelyFinal(sym1) // initialization important
+      || isEffectivelyFinal(sym2)
       || !sym1.isTrait && !sym2.isTrait
       || sym1.isSealed && sym2.isSealed && allChildrenAreIrreconcilable(sym1, sym2) && !currentRun.compiles(sym1) && !currentRun.compiles(sym2)
     )
+    private def isEffectivelyFinal(sym: Symbol): Boolean = (
+      // initialization important
+      sym.initialize.isEffectivelyFinal || (
+        settings.future.value && isTupleSymbol(sym) // SI-7294 step into the future and treat TupleN as final.
+      )
+    )
+
     def isNeverSubClass(sym1: Symbol, sym2: Symbol) = areIrreconcilableAsParents(sym1, sym2)
 
     private def isNeverSubArgs(tps1: List[Type], tps2: List[Type], tparams: List[Symbol]): Boolean = /*logResult(s"isNeverSubArgs($tps1, $tps2, $tparams)")*/ {
diff --git a/test/files/neg/t7294.check b/test/files/neg/t7294.check
new file mode 100644
index 0000000000..f15289c1c0
--- /dev/null
+++ b/test/files/neg/t7294.check
@@ -0,0 +1,6 @@
+t7294.scala:4: warning: fruitless type test: a value of type (Int, Int) cannot also be a Seq[A]
+  (1, 2) match { case Seq() => 0; case _ => 1 }
+                         ^
+error: No warnings can be incurred under -Xfatal-warnings.
+one warning found
+one error found
diff --git a/test/files/neg/t7294.flags b/test/files/neg/t7294.flags
new file mode 100644
index 0000000000..3f3381a45b
--- /dev/null
+++ b/test/files/neg/t7294.flags
@@ -0,0 +1 @@
+-Xfuture -Xfatal-warnings
diff --git a/test/files/neg/t7294.scala b/test/files/neg/t7294.scala
new file mode 100644
index 0000000000..335d071124
--- /dev/null
+++ b/test/files/neg/t7294.scala
@@ -0,0 +1,5 @@
+object Test {
+  // Treat TupleN as final under -Xfuture for the for the purposes
+  // of the "fruitless type test" warning.
+  (1, 2) match { case Seq() => 0; case _ => 1 }
+}
diff --git a/test/files/pos/t7294.scala b/test/files/pos/t7294.scala
new file mode 100644
index 0000000000..ccac2b1400
--- /dev/null
+++ b/test/files/pos/t7294.scala
@@ -0,0 +1,6 @@
+object Test {
+  // no fruitless warning as Tuple2 isn't (yet) final.
+  // The corresponding `neg` test will treat it as final
+  // for the purposes of these tests under -Xfuture.
+  (1, 2) match { case Seq() => 0; case _ => 1 }
+}