Handle & and | types when computing tuple arity

Fixes scala#15302

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

@@ -62,10 +62,13 @@ object TypeUtils {
         if self.termSymbol == defn.EmptyTupleModule then 0 else -1
       case self: TypeRef if relaxEmptyTuple && self.classSymbol == defn.EmptyTupleModule.moduleClass =>
         0
-      case self if defn.isTupleClass(self.classSymbol) =>
-        self.dealias.argInfos.length
+      case self: AndOrType =>
+        val arity1 = self.tp1.tupleArity(relaxEmptyTuple)
+        val arity2 = self.tp2.tupleArity(relaxEmptyTuple)
+        if arity1 == arity2 then arity1 else -1
       case _ =>
-        -1
+        if defn.isTupleClass(self.classSymbol) then self.dealias.argInfos.length
+        else -1
     }
 
     /** The element types of this tuple type, which can be made up of EmptyTuple, TupleX and `*:` pairs */

tests/run/i15302a.scala

@@ -0,0 +1,6 @@
+inline def flatConcat2[A, B](a: A, b: B) =
+  b match
+    case bs: *:[bh, bt]  => a *: bs
+
+@main def Test =
+  val x: (Int, Int, Int) = flatConcat2(1, (2,3))

tests/run/i15302b.scala

@@ -0,0 +1,12 @@
+@main def Test =
+  val tup2: Any = (1, 2)
+  val x1: (Int, Int) = tup2.asInstanceOf[(Int *: Int *: EmptyTuple) & (Int, Int)]
+  val x2: (Int, Int) = tup2.asInstanceOf[(Int *: Int *: Tuple) & (Int, Int)]
+  val x3: (Int, Int) = tup2.asInstanceOf[(Int *: Int *: EmptyTuple) | (Int, Int)]
+  val x4: Tuple = tup2.asInstanceOf[(Int *: Int *: Tuple) | (Int, Int)]
+
+  val tup3: Any = (1, 2, 3)
+  val x5: (Int, Int, Int) = tup3.asInstanceOf[Int *: ((Int *: Int *: EmptyTuple) & (Int, Int))]
+  val x6: (Int, Int, Int) = tup3.asInstanceOf[Int *: ((Int *: Int *: Tuple) & (Int, Int))]
+  val x7: (Int, Int, Int) = tup3.asInstanceOf[Int *: ((Int *: Int *: EmptyTuple) | (Int, Int))]
+  val x8: Tuple = tup3.asInstanceOf[Int *: ((Int *: Int *: Tuple) | (Int, Int))]