Fix reachability of unapplySeq of non-List sequences

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -371,10 +371,15 @@ class SpaceEngine(using Context) extends SpaceLogic {
       val fun1 = funPart(fun)
       val funRef = fun1.tpe.asInstanceOf[TermRef]
       if (fun.symbol.name == nme.unapplySeq)
-        if (fun.symbol.owner == scalaSeqFactoryClass)
+        val (arity, elemTp, resultTp) = unapplySeqInfo(fun.tpe.widen.finalResultType, fun.srcPos)
+        if (fun.symbol.owner == scalaSeqFactoryClass && scalaListType.appliedTo(elemTp) <:< pat.tpe)
+          // The exhaustivity and reachability logic already handles decomposing sum types (into its subclasses)
+          // and product types (into its components).  To get better counter-examples for patterns that are of type
+          // List (or a super-type of list, like LinearSeq) we project them into spaces that use `::` and Nil.
+          // Doing so with a pattern of `case Seq() =>` with a scrutinee of type `Vector()` doesn't work because the
+          // space is then discarded leading to a false positive reachability warning, see #13931.
           projectSeq(pats)
         else {
-          val (arity, elemTp, resultTp) = unapplySeqInfo(fun.tpe.widen.finalResultType, fun.srcPos)
           if (elemTp.exists)
             Prod(erase(pat.tpe.stripAnnots, isValue = false), funRef, projectSeq(pats) :: Nil)
           else
@@ -959,9 +964,8 @@ class SpaceEngine(using Context) extends SpaceLogic {
       if prev == Empty && covered == Empty then // defer until a case is reachable
         deferred ::= pat
       else {
-        // FIXME: These should be emitted, but reverted for i13931
-        //for (pat <- deferred.reverseIterator)
-        //  report.warning(MatchCaseUnreachable(), pat.srcPos)
+        for (pat <- deferred.reverseIterator)
+          report.warning(MatchCaseUnreachable(), pat.srcPos)
         if pat != EmptyTree // rethrow case of catch uses EmptyTree
             && isSubspace(covered, prev)
         then {

compiler/test/dotty/tools/dotc/transform/PatmatExhaustivityTest.scala

@@ -66,7 +66,7 @@ class PatmatExhaustivityTest {
       .filter(f => f.extension == "scala" || f.isDirectory)
       .filter { f =>
         val path = if f.isDirectory then f.path + "/" else f.path
-        path.contains(Properties.testsFilter.getOrElse(""))
+        Properties.testsFilter.getOrElse("").split(',').exists(path.contains)
       }
       .map(f => if f.isDirectory then compileDir(f.jpath) else compileFile(f.jpath))
 

tests/neg-custom-args/fatal-warnings/i8711.check

@@ -1,8 +1,8 @@
--- Error: tests/neg-custom-args/fatal-warnings/i8711.scala:7:9 ---------------------------------------------------------
+-- [E030] Match case Unreachable Error: tests/neg-custom-args/fatal-warnings/i8711.scala:7:9 ---------------------------
 7 |    case x: B => x // error: this case is unreachable since class A is not a subclass of class B
-  |         ^
-  |         this case is unreachable since type A and class B are unrelated
--- Error: tests/neg-custom-args/fatal-warnings/i8711.scala:12:9 --------------------------------------------------------
+  |         ^^^^
+  |         Unreachable case
+-- [E030] Match case Unreachable Error: tests/neg-custom-args/fatal-warnings/i8711.scala:12:9 --------------------------
 12 |    case x: C => x // error
-   |         ^
-   |         this case is unreachable since type A | B and class C are unrelated
+   |         ^^^^
+   |         Unreachable case

tests/patmat/i13485.check

@@ -1 +1,2 @@
+11: Match case Unreachable
 16: Match case Unreachable

tests/patmat/i13485.scala

@@ -8,7 +8,7 @@ sealed trait Foo
 class Bar
 
 def test1(bar: Bar) = bar match
-  case _: Foo => 1 // FIXME: this is unreachable, but reverted for i13931
+  case _: Foo => 1
   case _: Bar => 2
 
 def test2(bar: Bar) = bar match

tests/patmat/i13931.scala

@@ -2,3 +2,6 @@ class Test:
   def test = Vector() match
     case Seq() => println("empty")
     case _ => println("non-empty")
+
+  def test2 = IndexedSeq() match { case IndexedSeq() => case _ => }
+  def test3 = IndexedSeq() match { case IndexedSeq(1) => case _ => }