Adapt generic tuples to be able to access members

Fixes #14215

Author: nicolasstucki

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

@@ -3854,6 +3854,12 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               gadts.println(i"Member selection healed by GADT approximation")
               tree.cast(gadtApprox)
             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
           else tree // other adaptations for selections are handled in typedSelect
         case _ if ctx.mode.is(Mode.ImplicitsEnabled) && tree.tpe.isValueType =>
           checkConversionsSpecific(pt, tree.srcPos)

tests/neg/genericTupleMembers.scala

@@ -0,0 +1,38 @@
+def Test: Unit =
+  val tup1 = 1 *: EmptyTuple
+  val tup2 = 1 *: 2 *: EmptyTuple
+  val tup3 = 1 *: 2 *: 3 *: EmptyTuple
+  val tup4 = 1 *: 2 *: 3 *: 4 *: EmptyTuple
+  val tup5 = 1 *: 2 *: 3 *: 4 *: 5 *: EmptyTuple
+  val tup22 = 1 *: 2 *: 3 *: 4 *: 5 *: 6 *: 7 *: 8 *: 9 *: 10 *: 11 *: 12 *: 13 *: 14 *: 15 *: 16 *: 17 *: 18 *: 19 *: 20 *: 21 *: 22 *: EmptyTuple
+  val tup23 = 1 *: 2 *: 3 *: 4 *: 5 *: 6 *: 7 *: 8 *: 9 *: 10 *: 11 *: 12 *: 13 *: 14 *: 15 *: 16 *: 17 *: 18 *: 19 *: 20 *: 21 *: 22 *: 23 *: EmptyTuple
+
+  tup1._2 // error
+
+  tup2._3 // error
+
+  tup22._23 // error
+
+  tup23._1 // error
+  tup23._2 // error
+  tup23._3 // error
+  tup23._4 // error
+  tup23._5 // error
+  tup23._6 // error
+  tup23._7 // error
+  tup23._8 // error
+  tup23._9 // error
+  tup23._10 // error
+  tup23._11 // error
+  tup23._12 // error
+  tup23._13 // error
+  tup23._14 // error
+  tup23._15 // error
+  tup23._16 // error
+  tup23._17 // error
+  tup23._18 // error
+  tup23._19 // error
+  tup23._20 // error
+  tup23._21 // error
+  tup23._22 // error
+  tup23._23 // error

tests/run/genericTupleMembers.scala

@@ -0,0 +1,46 @@
+
+@main def Test: Unit =
+  val tup1 = 1 *: EmptyTuple
+  val tup2 = 1 *: 2 *: EmptyTuple
+  val tup3 = 1 *: 2 *: 3 *: EmptyTuple
+  val tup4 = 1 *: 2 *: 3 *: 4 *: EmptyTuple
+  val tup5 = 1 *: 2 *: 3 *: 4 *: 5 *: EmptyTuple
+  val tup22 = 1 *: 2 *: 3 *: 4 *: 5 *: 6 *: 7 *: 8 *: 9 *: 10 *: 11 *: 12 *: 13 *: 14 *: 15 *: 16 *: 17 *: 18 *: 19 *: 20 *: 21 *: 22 *: EmptyTuple
+
+  tup1._1
+
+  tup2._1
+  tup2._2
+  tup2.swap
+
+  tup3._1
+  tup3._2
+  tup3._3
+
+  tup4._1
+  tup4._2
+  tup4._3
+  tup4._4
+
+  tup22._1
+  tup22._2
+  tup22._3
+  tup22._4
+  tup22._5
+  tup22._6
+  tup22._7
+  tup22._8
+  tup22._9
+  tup22._10
+  tup22._11
+  tup22._12
+  tup22._13
+  tup22._14
+  tup22._15
+  tup22._16
+  tup22._17
+  tup22._18
+  tup22._19
+  tup22._20
+  tup22._21
+  tup22._22

tests/run/i14215.scala

@@ -0,0 +1,12 @@
+def f[T <: Tuple2[Int, Int]](tup: T): T = tup
+
+@main def Test: Unit =
+  (1, 2)._1
+  f((1, 2))._1
+
+  (1 *: 2 *: EmptyTuple)._1
+  f(1 *: 2 *: EmptyTuple)._1
+  f[Int *: Int *: EmptyTuple](1 *: 2 *: EmptyTuple)._1
+
+  f[Int *: Int *: EmptyTuple]((1, 2))._1
+  f[Tuple2[Int, Int]](1 *: 2 *: EmptyTuple)._1