Short-circuit isCheckable with classSymbol

A mutually recursive, and ever-growing type pair like ExprMap and EM (in
the test case) would blow up isCheckable.  Let's use the classSymbol
instead.

[Cherry-picked eff3424]

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

@@ -775,7 +775,7 @@ object SpaceEngine {
   }
 
   private def exhaustivityCheckable(sel: Tree)(using Context): Boolean = {
-    val seen = collection.mutable.Set.empty[Type]
+    val seen = collection.mutable.Set.empty[Symbol]
 
     // Possible to check everything, but be compatible with scalac by default
     def isCheckable(tp: Type): Boolean =
@@ -791,7 +791,7 @@ object SpaceEngine {
       tpw.isRef(defn.BooleanClass) ||
       classSym.isAllOf(JavaEnumTrait) ||
       classSym.is(Case) && {
-        if seen.add(tpw) then productSelectorTypes(tpw, sel.srcPos).exists(isCheckable(_))
+        if seen.add(classSym) then productSelectorTypes(tpw, sel.srcPos).exists(isCheckable(_))
         else true // recursive case class: return true and other members can still fail the check
       }
 

tests/pos/i19433.scala

@@ -0,0 +1,12 @@
+// minimised from github.com/Adam-Vandervorst/CZ2
+
+import scala.collection.mutable
+
+private trait EMImpl[V, F[_]]
+
+case class EM[V2](apps: ExprMap[ExprMap[V2]]) extends EMImpl[V2, EM]:
+  def collect[W](pf: PartialFunction[V2, W]): Unit =
+    val apps1 = apps.collect(_.collect(pf))
+
+case class ExprMap[V](var em: EM[V] = null) extends EMImpl[V, ExprMap]:
+  def collect[W](pf: PartialFunction[V, W]): ExprMap[W] = ??? // was: StackOverflow in isCheckable