SI-7183 Disable unreachability for withFilter matches.

This avoids spurious unreachable warnings on code
that the user didn't write.

The parser desugars for-comprehensions such as:

    for (A(a) <- List(new A)) yield a

To:

    List(new A()).withFilter(((check$ifrefutable$2) =>
      check$ifrefutable$2: @scala.unhecked match {
       case A((a @ _)) => true
       case _ => false
      })
    )

But, if `A.unapply` returns `Some[_]`, the last case is dead code.
(Matching against a regular case class *would* fall through in
the caes of a null scrutinee.)

In SI-6902, we enabled unreachability warnings, even if the
scrutinee was annotated as @unchecked. That was consistent
with the 2.9.2 behaviour, it was only disabled temporarily
(actually, accidentally) in 2.10.0. But, the old pattern matcher
didn't warn about this code.

This commit makes the pattern matcher recognise the special
scrutinee based on its name and disables both exhaustivity
*and* unreachability analysis.

To do so, the we generalize the boolean flag `unchecked` to
the class `Suppression`.

3 files changed, 42 insertions, 14 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
index f7579ad249..87fe3fb3f6 100644
--- a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
@@ -936,7 +936,7 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
 // the making of the trees
 ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
   trait TreeMakers extends TypedSubstitution { self: CodegenCore =>
-    def optimizeCases(prevBinder: Symbol, cases: List[List[TreeMaker]], pt: Type, unchecked: Boolean): (List[List[TreeMaker]], List[Tree]) =
+    def optimizeCases(prevBinder: Symbol, cases: List[List[TreeMaker]], pt: Type, supression: Suppression): (List[List[TreeMaker]], List[Tree]) =
       (cases, Nil)
 
     def emitSwitch(scrut: Tree, scrutSym: Symbol, cases: List[List[TreeMaker]], pt: Type, matchFailGenOverride: Option[Tree => Tree], unchecked: Boolean): Option[Tree] =
@@ -1408,18 +1408,24 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
         def matchFailGen = (matchFailGenOverride orElse Some(CODE.MATCHERROR(_: Tree)))
         patmatDebug("combining cases: "+ (casesNoSubstOnly.map(_.mkString(" >> ")).mkString("{", "\n", "}")))
 
-        val (unchecked, requireSwitch) =
-          if (settings.XnoPatmatAnalysis.value) (true, false)
+        val (suppression, requireSwitch): (Suppression, Boolean) =
+          if (settings.XnoPatmatAnalysis.value) (Suppression.NoSuppresion, false)
           else scrut match {
-            case Typed(_, tpt) =>
-              (tpt.tpe hasAnnotation UncheckedClass,
-               // matches with two or fewer cases need not apply for switchiness (if-then-else will do)
-               treeInfo.isSwitchAnnotation(tpt.tpe) && casesNoSubstOnly.lengthCompare(2) > 0)
+            case Typed(tree, tpt) =>
+              val suppressExhaustive = tpt.tpe hasAnnotation UncheckedClass
+              val supressUnreachable = tree match {
+                case Ident(name) if name startsWith nme.CHECK_IF_REFUTABLE_STRING => true // SI-7183 don't warn for withFilter's that turn out to be irrefutable.
+                case _ => false
+              }
+              val suppression = Suppression(suppressExhaustive, supressUnreachable)
+              // matches with two or fewer cases need not apply for switchiness (if-then-else will do)
+              val requireSwitch = treeInfo.isSwitchAnnotation(tpt.tpe) && casesNoSubstOnly.lengthCompare(2) > 0
+              (suppression, requireSwitch)
             case _ =>
-              (false, false)
+              (Suppression.NoSuppresion, false)
           }
 
-        emitSwitch(scrut, scrutSym, casesNoSubstOnly, pt, matchFailGenOverride, unchecked).getOrElse{
+        emitSwitch(scrut, scrutSym, casesNoSubstOnly, pt, matchFailGenOverride, suppression.exhaustive).getOrElse{
           if (requireSwitch) typer.context.unit.warning(scrut.pos, "could not emit switch for @switch annotated match")
 
           if (casesNoSubstOnly nonEmpty) {
@@ -1436,7 +1442,7 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
                   }) None
               else matchFailGen
 
-            val (cases, toHoist) = optimizeCases(scrutSym, casesNoSubstOnly, pt, unchecked)
+            val (cases, toHoist) = optimizeCases(scrutSym, casesNoSubstOnly, pt, suppression)
 
             val matchRes = codegen.matcher(scrut, scrutSym, pt)(cases map combineExtractors, synthCatchAll)
 
@@ -3831,11 +3837,13 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
                               with OptimizedCodegen
                               with SymbolicMatchAnalysis
                               with DPLLSolver { self: TreeMakers =>
-    override def optimizeCases(prevBinder: Symbol, cases: List[List[TreeMaker]], pt: Type, unchecked: Boolean): (List[List[TreeMaker]], List[Tree]) = {
-      unreachableCase(prevBinder, cases, pt) foreach { caseIndex =>
-        reportUnreachable(cases(caseIndex).last.pos)
+    override def optimizeCases(prevBinder: Symbol, cases: List[List[TreeMaker]], pt: Type, suppression: Suppression): (List[List[TreeMaker]], List[Tree]) = {
+      if (!suppression.unreachable) {
+        unreachableCase(prevBinder, cases, pt) foreach { caseIndex =>
+          reportUnreachable(cases(caseIndex).last.pos)
+        }
       }
-      if (!unchecked) {
+      if (!suppression.exhaustive) {
         val counterExamples = exhaustive(prevBinder, cases, pt)
         if (counterExamples.nonEmpty)
           reportMissingCases(prevBinder.pos, counterExamples)
@@ -3860,3 +3868,9 @@ object PatternMatchingStats {
   val patmatAnaExhaust    = Statistics.newSubTimer  ("  of which in exhaustivity", patmatNanos)
   val patmatAnaReach      = Statistics.newSubTimer  ("  of which in unreachability", patmatNanos)
 }
+
+final case class Suppression(exhaustive: Boolean, unreachable: Boolean)
+
+object Suppression {
+  val NoSuppresion = Suppression(false, false)
+}
\ No newline at end of file
diff --git a/test/files/pos/t7183.flags b/test/files/pos/t7183.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/pos/t7183.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/pos/t7183.scala b/test/files/pos/t7183.scala
new file mode 100644
index 0000000000..7647c1634b
--- /dev/null
+++ b/test/files/pos/t7183.scala
@@ -0,0 +1,13 @@
+class A
+object A {
+  def unapply(a: A): Some[A] = Some(a) // Change return type to Option[A] and the warning is gone
+}
+
+object Test {
+  for (A(a) <- List(new A)) yield a // spurious dead code warning.
+}
+
+// List(new A()).withFilter(((check$ifrefutable$2) => check$ifrefutable$2: @scala.unchecked match {
+//  case A((a @ _)) => true
+//  case _ => false // this is dead code, but it's compiler generated.
+// }))