Fix missing case in isSubspace, which broke reachablility (#18326)

Author: dwijnand

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

@@ -193,10 +193,11 @@ object SpaceEngine {
         || canDecompose(b) && isSubspace(a, Or(decompose(b)))
       case (Prod(tp1, _, _), Typ(tp2, _)) =>
         isSubType(tp1, tp2)
-      case (Typ(tp1, _), Prod(tp2, fun, ss)) =>
+      case (a @ Typ(tp1, _), Prod(tp2, fun, ss)) =>
         isSubType(tp1, tp2)
         && covers(fun, tp1, ss.length)
         && isSubspace(Prod(tp2, fun, signature(fun, tp1, ss.length).map(Typ(_, false))), b)
+        || canDecompose(a) && isSubspace(Or(decompose(a)), b)
       case (Prod(_, fun1, ss1), Prod(_, fun2, ss2)) =>
         isSameUnapply(fun1, fun2) && ss1.lazyZip(ss2).forall(isSubspace)
     }
@@ -597,7 +598,7 @@ object SpaceEngine {
   }
 
   /** Whether the extractor covers the given type */
-  def covers(unapp: TermRef, scrutineeTp: Type, argLen: Int)(using Context): Boolean =
+  def covers(unapp: TermRef, scrutineeTp: Type, argLen: Int)(using Context): Boolean = trace(i"covers($unapp, $scrutineeTp, $argLen)") {
     SpaceEngine.isIrrefutable(unapp, argLen)
     || unapp.symbol == defn.TypeTest_unapply && {
       val AppliedType(_, _ :: tp :: Nil) = unapp.prefix.widen.dealias: @unchecked
@@ -607,6 +608,7 @@ object SpaceEngine {
       val AppliedType(_, tp :: Nil) = unapp.prefix.widen.dealias: @unchecked
       scrutineeTp <:< tp
     }
+  }
 
   /** Decompose a type into subspaces -- assume the type can be decomposed */
   def decompose(tp: Type)(using Context): List[Type] = trace(i"decompose($tp)", debug) {

tests/patmat/i18118.check

@@ -0,0 +1,4 @@
+12: Pattern Match
+21: Pattern Match
+32: Pattern Match
+41: Pattern Match

tests/patmat/i18118.scala

@@ -0,0 +1,41 @@
+// scalac: -Werror
+
+object O1:
+  sealed trait A
+  case class B() extends A
+  case class C() extends A
+
+
+  def bigMatch(x: A) = x match
+    case B() =>
+    case C() =>
+    case _ => // error
+
+object O2:
+  sealed trait A
+  case class B() extends A
+
+
+  def bigMatch(x: A) = x match
+    case B() =>
+    case _ => // error // was: no "unreachable but for null" warning
+
+object O3:
+  sealed trait A
+  case class B() extends A
+  case class C() extends A
+
+
+  def bigMatch(x: A) = x match
+    case _: B =>
+    case _: C =>
+    case _ => // error
+
+object O4:
+  sealed trait A
+  case class B() extends A
+
+
+  def bigMatch(x: A) = x match
+    case _: B =>
+    case _ => // error