Fix isTupleNType, which is documented to act on the dealiased type

Author: dwijnand

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

@@ -1490,8 +1490,9 @@ 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
-    arity <= MaxTupleArity && TupleType(arity) != null && tp.isRef(TupleType(arity).symbol)
+    val tp1 = tp.dealias
+    val arity = tp1.argInfos.length
+    arity <= MaxTupleArity && TupleType(arity) != null && tp1.isRef(TupleType(arity).symbol)
   }
 
   def tupleType(elems: List[Type]): Type = {

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, ""))