WIP: resolves lampepfl#13737

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

@@ -380,7 +380,12 @@ class SpaceEngine(using Context) extends SpaceLogic {
             Prod(erase(pat.tpe.stripAnnots, isValue = false), funRef, pats.take(arity - 1).map(project) :+ projectSeq(pats.drop(arity - 1)))
         }
       else
-        Prod(erase(pat.tpe.stripAnnots, isValue = false), funRef, pats.map(project))
+        val tp = fun.tpe.widen.finalResultType
+        val isProdMatch = isProductMatch(tp, pats.length) || defn.isProductSubType(tp) && tp.classSymbol.is(Scala2x)
+        if isProdMatch then
+          Prod(erase(pat.tpe.stripAnnots, isValue = false), funRef, pats.map(project))
+        else
+          Typ(erase(pat.tpe.stripAnnots, isValue = false), decomposed = false)
 
     case Typed(pat @ UnApply(_, _, _), _) =>
       project(pat)

tests/patmat/irrefutable.mini.check

@@ -0,0 +1 @@
+15: Pattern Match Exhaustivity: ExM(_, _)

tests/patmat/irrefutable.mini.scala

@@ -0,0 +1,18 @@
+sealed class M(n: Int, s: String) extends Product {
+  def _1: Int = n
+  def _2: String = s
+  def isEmpty: Boolean = s.size > n
+  def get: M = this
+
+  def canEqual(that: Any): Boolean = true
+  def productArity: Int = 2
+  def productElement(n: Int): Any = ???
+}
+
+object ExM {
+  def unapply(m: M): M = m
+
+  def test(m: M) = m match {  // warning
+    case ExM(s) =>
+  }
+}

tests/pos/i13737.scala

@@ -0,0 +1,18 @@
+sealed trait Result
+
+case class Success(result: String, next: Int) extends Result {
+  def isEmpty: Boolean = 10 % 2 == 1
+  def get: String = result
+}
+
+object Success {
+  def unapply(x: Success): Success = x
+}
+
+def main =
+  val res: Result = ???
+  res match
+    case Success(v) => v
+
+
+// todo: check why only sealed trait interference