Exhaustivity warnings on nested case classes

Fixes scala#13003, by refixing scala#12485 (PR scala#12488).

Part of the issue is that isCheckable behaves differently under
-Ycheck-all-patmat and our tests only run under that flag.  So for
starters I added a test variant where that flag isn't used.  I'd like to
understand why that flag exists to see if we could remove it from
guarding the logic and the tests.

Also make sure that any patmat test that throws an exception fails the
test...

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

@@ -804,6 +804,8 @@ class SpaceEngine(using Context) extends SpaceLogic {
   }
 
   private def exhaustivityCheckable(sel: Tree): Boolean = {
+    val seen = collection.mutable.Set.empty[Type]
+
     // Possible to check everything, but be compatible with scalac by default
     def isCheckable(tp: Type): Boolean =
       val tpw = tp.widen.dealias
@@ -815,16 +817,15 @@ class SpaceEngine(using Context) extends SpaceLogic {
         isCheckable(and.tp1) || isCheckable(and.tp2)
       }) ||
       tpw.isRef(defn.BooleanClass) ||
-      classSym.isAllOf(JavaEnumTrait)
+      classSym.isAllOf(JavaEnumTrait) ||
+      classSym.is(Case) && {
+        if seen.add(tpw) then productSelectorTypes(tpw, sel.srcPos).exists(isCheckable(_))
+        else true // recursive case class: return true and other members can still fail the check
+      }
 
     val res = !sel.tpe.hasAnnotation(defn.UncheckedAnnot) && {
       ctx.settings.YcheckAllPatmat.value
       || isCheckable(sel.tpe)
-      || {
-        val tpw = sel.tpe.widen.dealias
-        val classSym = tpw.classSymbol
-        classSym.is(Case) && productSelectorTypes(tpw, sel.srcPos).exists(isCheckable(_))
-      }
     }
 
     debug.println(s"exhaustivity checkable: ${sel.show} = $res")

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

@@ -15,21 +15,26 @@ import java.nio.file.{Path => JPath}
 import scala.io.Source._
 import org.junit.Test
 
+class PatmatDefaultExhaustivityTest extends PatmatExhaustivityTest {
+  override val testsDir = "tests/patmat-default"
+  override val options  = super.options.filter(_ != "-Ycheck-all-patmat")
+}
+
 class PatmatExhaustivityTest {
   val testsDir = "tests/patmat"
   // stop-after: patmatexhaust-huge.scala crash compiler
-  val options = List("-pagewidth", "80", "-color:never", "-Ystop-after:explicitSelf", "-Ycheck-all-patmat", "-classpath", TestConfiguration.basicClasspath)
+  def options = List("-pagewidth", "80", "-color:never", "-Ystop-after:explicitSelf", "-Ycheck-all-patmat", "-classpath", TestConfiguration.basicClasspath)
 
   private def compile(files: Seq[String]): Seq[String] = {
     val stringBuffer = new StringWriter()
-    val reporter = TestReporter.simplifiedReporter(new PrintWriter(stringBuffer))
+    val printWriter  = new PrintWriter(stringBuffer)
+    val reporter = TestReporter.simplifiedReporter(printWriter)
 
     try {
       Main.process((options ++ files).toArray, reporter, null)
     } catch {
       case e: Throwable =>
-        println(s"Compile $files exception:")
-        e.printStackTrace()
+        e.printStackTrace(printWriter)
     }
 
     stringBuffer.toString.trim.replaceAll("\\s+\n", "\n") match {

tests/patmat-default/i13003.check

@@ -0,0 +1,4 @@
+4: Pattern Match Exhaustivity: One(Two(None))
+7: Pattern Match Exhaustivity: Two(None)
+10: Pattern Match Exhaustivity: None, Some(None)
+13: Pattern Match Exhaustivity: None, Some(None), Some(Some(None))

tests/patmat-default/i13003.scala

@@ -0,0 +1,14 @@
+case class One(two: Two)
+case class Two(o: Option[Int])
+
+def matchOneTwo(one: One) = one match
+    case One(Two(Some(i))) => "match!"
+
+def matchTwo(two: Two) = two match
+    case Two(Some(i)) => "match!"
+
+def matchOO(oo: Option[Option[Int]]) = oo match
+    case Some(Some(i)) => "match!"
+
+def matchOOO(ooo: Option[Option[Option[Int]]]) = ooo match
+    case Some(Some(Some(i))) => "match!"