Revert to fix false positive reachability warnings

Author: dwijnand

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

@@ -959,8 +959,9 @@ class SpaceEngine(using Context) extends SpaceLogic {
       if prev == Empty && covered == Empty then // defer until a case is reachable
         deferred ::= pat
       else {
-        for (pat <- deferred.reverseIterator)
-          report.warning(MatchCaseUnreachable(), pat.srcPos)
+        // FIXME: These should be emitted, but reverted for i13931
+        //for (pat <- deferred.reverseIterator)
+        //  report.warning(MatchCaseUnreachable(), pat.srcPos)
         if pat != EmptyTree // rethrow case of catch uses EmptyTree
             && isSubspace(covered, prev)
         then {

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

@@ -1,8 +1,8 @@
--- [E030] Match case Unreachable Error: tests/neg-custom-args/fatal-warnings/i8711.scala:7:9 ---------------------------
+-- 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
-  |         ^^^^
-  |         Unreachable case
--- [E030] Match case Unreachable Error: tests/neg-custom-args/fatal-warnings/i8711.scala:12:9 --------------------------
+  |         ^
+  |         this case is unreachable since type A and class B are unrelated
+-- Error: tests/neg-custom-args/fatal-warnings/i8711.scala:12:9 --------------------------------------------------------
 12 |    case x: C => x // error
-   |         ^^^^
-   |         Unreachable case
+   |         ^
+   |         this case is unreachable since type A | B and class C are unrelated

tests/patmat/i13485.check

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

tests/patmat/i13931.scala

@@ -0,0 +1,4 @@
+class Test:
+  def test = Vector() match
+    case Seq() => println("empty")
+    case _ => println("non-empty")