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.

Author: dwijnand

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

@@ -804,8 +804,9 @@ class SpaceEngine(using Context) extends SpaceLogic {
   }
 
   private def exhaustivityCheckable(sel: Tree): Boolean = {
+    val cache = collection.mutable.Map.empty[Type, Boolean]
     // Possible to check everything, but be compatible with scalac by default
-    def isCheckable(tp: Type): Boolean =
+    def isCheckable(tp: Type): Boolean = cache.getOrElseUpdate(tp, {
       val tpw = tp.widen.dealias
       val classSym = tpw.classSymbol
       classSym.is(Sealed) ||
@@ -815,16 +816,13 @@ 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) && productSelectorTypes(tpw, sel.srcPos).exists(isCheckable(_))
+    })
 
     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

@@ -15,10 +15,15 @@ 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()

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!"