Fix casting of generic tuple selection

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1489,10 +1489,11 @@ class Definitions {
    * @return true if the dealiased type of `tp` is `TupleN[T1, T2, ..., Tn]`
    */
   def isTupleNType(tp: Type)(using Context): Boolean = {
-    val arity = tp.dealias.argInfos.length
+    val tp1 = tp.dealias
+    val arity = tp1.argInfos.length
     arity <= MaxTupleArity && {
       val tupletp = TupleType(arity)
-      tupletp != null && tp.isRef(tupletp.symbol)
+      tupletp != null && tp1.isRef(tupletp.symbol)
     }
   }
 

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -9,6 +9,7 @@ import Symbols._, Contexts._, Types._, StdNames._, NameOps._
 import util.Spans._
 import typer.Applications.*
 import SymUtils._
+import TypeUtils.*
 import Flags._, Constants._
 import Decorators._
 import NameKinds.{PatMatStdBinderName, PatMatAltsName, PatMatResultName}
@@ -332,11 +333,11 @@ object PatternMatcher {
           ref(defn.RuntimeTuplesModule)
             .select(defn.RuntimeTuples_apply)
             .appliedTo(receiver, Literal(Constant(i)))
-            .cast(args(i).tpe.widen)
 
         if (isSyntheticScala2Unapply(unapp.symbol) && caseAccessors.length == args.length)
           def tupleSel(sym: Symbol) = ref(scrutinee).select(sym)
-          val isGenericTuple = defn.isTupleClass(caseClass) && !defn.isTupleNType(tree.tpe)
+          val isGenericTuple = defn.isTupleClass(caseClass) &&
+            !defn.isTupleNType(tree.tpe match { case tp: OrType => tp.join case tp => tp }) // widen even hard unions, to see if it's a union of tuples
           val components = if isGenericTuple then caseAccessors.indices.toList.map(tupleApp(_, ref(scrutinee))) else caseAccessors.map(tupleSel)
           matchArgsPlan(components, args, onSuccess)
         else if (unapp.tpe <:< (defn.BooleanType))

tests/run/i14587.min.scala

@@ -0,0 +1,10 @@
+object Test:
+  def test(cond: Boolean) =
+    val tup: (String, Unit) | (Int, Unit) = if cond then ("", ()) else (1, ())
+    tup match
+      case (s: String, _) => s
+      case _ => "n/a"
+
+  def main(args: Array[String]): Unit =
+    test(true)  // works
+    test(false) // was: ClassCastException: class scala.None$ cannot be cast to class scala.Some

tests/run/i14587.opaques.scala

@@ -0,0 +1,8 @@
+object Test:
+  opaque type Tup = (Int, String)
+
+  def test(tup: Tup) =
+    val (n, s) = tup
+    assert(n == 1 && s == "")
+
+  def main(args: Array[String]): Unit = test((1, ""))

tests/run/i14587.scala

@@ -0,0 +1,16 @@
+def test(foo: String): Unit = {
+  // Does not crash if the type is written explicitly as: Option[(Option[Int], String)]
+  val bar = {
+    if (foo.isEmpty) Some((Some(1), ""))
+    else Some((None, ""))
+  }
+
+  bar.foreach {
+    case (Some(_), "") =>
+    case _ =>
+  }
+}
+
+@main def Test() =
+  test("") // works
+  test("a")