Fix tuple casting

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3943,10 +3943,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             else tree
           else if tree.tpe.derivesFrom(defn.PairClass) && !defn.isTupleNType(tree.tpe.widenDealias) then
             // If this is a generic tuple we need to cast it to make the TupleN/ members accessible.
-            // This only works for generic tuples of know size up to 22.
-            defn.tupleTypes(tree.tpe.widenTermRefExpr, Definitions.MaxTupleArity) match
-              case Some(elems) => tree.cast(defn.tupleType(elems))
-              case None => tree
+            // This works only for generic tuples of known size up to 22.
+            defn.tupleTypes(tree.tpe.widenTermRefExpr) match
+              case Some(elems) if elems.length <= Definitions.MaxTupleArity =>
+                tree.cast(defn.tupleType(elems))
+              case _ => tree
           else tree // other adaptations for selections are handled in typedSelect
         case _ if ctx.mode.is(Mode.ImplicitsEnabled) && tree.tpe.isValueType =>
           if pt.isRef(defn.AnyValClass, skipRefined = false)

tests/pos/i16104.scala

@@ -0,0 +1,18 @@
+trait JsonVal
+
+val value = ("a", "b", "c", "d", "e", "f", "g", "h", "i", "j", 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, "k", "l", "m", "n", "o", "p", "q")
+val tree: JsonVal = ???
+
+def Case2 = {
+  sealed trait Write[T]
+  object WriteOf:
+    final inline def tuple[T <: Tuple]: Write[T] = ???
+
+  given EntryToJson[T]: scala.Conversion[T, JsonStructureEntry[T]] = ???
+  class JsonStructureEntry[T](t: T):
+    def writeAs[X >: T](using Write[X]): util.Try[JsonVal] = ???
+
+  value
+  .writeAs(using WriteOf.tuple[String *: String *: String *: String *: String *: String *: String *: String *: String *: String *: Int *: Int *: Int *: Int *: Int *: Int *: Int *: Int *: Int *: Int *: String *: String *: String *: String *: String *: String *: String *: EmptyTuple])
+  .fold(_ => ???, _ == tree)
+}