Properly dealias tuple types when specializing (#18724)

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

@@ -37,15 +37,14 @@ class SpecializeTuples extends MiniPhase:
   end transformApply
 
   override def transformSelect(tree: Select)(using Context): Tree = tree match
-    case Select(qual, nme._1) if isAppliedSpecializableTuple(qual.tpe.widen) =>
-      Select(qual, nme._1.specializedName(qual.tpe.widen.argInfos.slice(0, 1)))
-    case Select(qual, nme._2) if isAppliedSpecializableTuple(qual.tpe.widen) =>
-      Select(qual, nme._2.specializedName(qual.tpe.widen.argInfos.slice(1, 2)))
+    case Select(qual, name @ (nme._1 | nme._2)) =>
+      qual.tpe.widenDealias match
+        case AppliedType(tycon, args) if defn.isSpecializableTuple(tycon.classSymbol, args) =>
+          val argIdx = if name == nme._1 then 0 else 1
+          Select(qual, name.specializedName(args(argIdx) :: Nil))
+        case _ =>
+          tree
     case _ => tree
-
-  private def isAppliedSpecializableTuple(tp: Type)(using Context) = tp match
-    case AppliedType(tycon, args) => defn.isSpecializableTuple(tycon.classSymbol, args)
-    case _                        => false
 end SpecializeTuples
 
 object SpecializeTuples:

tests/run/i18638.scala

@@ -0,0 +1,18 @@
+type U[H, T] = (Unit, Unit)
+object O:
+  opaque type U[H, T] <: (Unit, Unit) = (Unit, Unit)
+  def u: U[Int, Int] = ((), ())
+
+
+def test1(u: (Unit, Unit)) = u._1
+def test2(u: U[Int, Int]) = u._1
+def test3(u: O.U[Int, Int]) = u._1
+def test4() =
+  (((), ()): U[Int, Int]) match
+    case ((), ()) => println("ok")
+
+@main def Test: Unit =
+  test1(((), ()))
+  test2(((), ()))
+  test3(O.u)
+  test4()