From 12e7c9e982c1317bcf251b535fd124c6d017005b Mon Sep 17 00:00:00 2001
From: Nicolas Stucki <nicolas.stucki@gmail.com>
Date: Mon, 4 Dec 2023 10:32:16 +0100
Subject: [PATCH] Introduce `Tuple.ReverseOnto` and use it in `Tuple.reverse`

---
 library/src/scala/Tuple.scala                    | 13 +++++--------
 tests/pos/TupleReverse.scala                     | 16 ++++++++++++++++
 tests/pos/TupleReverseOnto.scala                 | 15 +++++++++++++++
 .../stdlibExperimentalDefinitions.scala          | 10 ++++------
 4 files changed, 40 insertions(+), 14 deletions(-)
 create mode 100644 tests/pos/TupleReverse.scala
 create mode 100644 tests/pos/TupleReverseOnto.scala

diff --git a/library/src/scala/Tuple.scala b/library/src/scala/Tuple.scala
index 6993b8202082..4f756f76db73 100644
--- a/library/src/scala/Tuple.scala
+++ b/library/src/scala/Tuple.scala
@@ -202,16 +202,13 @@ object Tuple {
 
   /** Type of the reversed tuple */
   @experimental
-  type Reverse[X <: Tuple] = Helpers.ReverseImpl[EmptyTuple, X]
+  type Reverse[X <: Tuple] = ReverseOnto[X, EmptyTuple]
 
+  /** Prepends all elements of a tuple in reverse order onto the other tuple */
   @experimental
-  object Helpers:
-
-    /** Type of the reversed tuple */
-    @experimental
-    type ReverseImpl[Acc <: Tuple, X <: Tuple] <: Tuple = X match
-      case x *: xs => ReverseImpl[x *: Acc, xs]
-      case EmptyTuple => Acc
+  type ReverseOnto[From <: Tuple, +To <: Tuple] <: Tuple = From match
+    case x *: xs => ReverseOnto[xs, x *: To]
+    case EmptyTuple => To
 
   /** Transforms a tuple `(T1, ..., Tn)` into `(T1, ..., Ti)`. */
   type Take[T <: Tuple, N <: Int] <: Tuple = N match {
diff --git a/tests/pos/TupleReverse.scala b/tests/pos/TupleReverse.scala
new file mode 100644
index 000000000000..9b83280afcf1
--- /dev/null
+++ b/tests/pos/TupleReverse.scala
@@ -0,0 +1,16 @@
+import scala.Tuple.*
+def test[T1, T2, T3, T4] =
+  summon[Reverse[EmptyTuple] =:= EmptyTuple]
+  summon[Reverse[T1 *: EmptyTuple] =:= T1 *: EmptyTuple]
+  summon[Reverse[(T1, T2)] =:= (T2, T1)]
+  summon[Reverse[(T1, T2, T3)] =:= (T3, T2, T1)]
+  summon[Reverse[(T1, T2, T3, T4)] =:= (T4, T3, T2, T1)]
+
+  summon[Reverse[(T1, T2, T3, T4)] =:= Reverse[(T1, T2, T3, T4)]]
+  summon[Reverse[(T1, T2, T3, T4)] <:< Reverse[(Any, Any, Any, Any)]]
+
+def test2[Tup <: Tuple] =
+  summon[Reverse[Tup] =:= Reverse[Tup]]
+
+def test3[T1, T2, T3, T4](tup1: (T1, T2, T3, T4)) =
+  summon[Reverse[tup1.type] =:= (T4, T3, T2, T1)]
diff --git a/tests/pos/TupleReverseOnto.scala b/tests/pos/TupleReverseOnto.scala
new file mode 100644
index 000000000000..bef2d0949b29
--- /dev/null
+++ b/tests/pos/TupleReverseOnto.scala
@@ -0,0 +1,15 @@
+import scala.Tuple.*
+def test[T1, T2, T3, T4] =
+  summon[ReverseOnto[(T1, T2), (T3, T4)] =:= ReverseOnto[(T1, T2), (T3, T4)]]
+  summon[ReverseOnto[(T1, T2), (T3, T4)] =:= (T2, T1, T3, T4)]
+  summon[ReverseOnto[(T1, T2), (T3, T4)] <:< (Any, Any, Any, Any)]
+  summon[ReverseOnto[(T1, T2), (T3, T4)] <:< ReverseOnto[(Any, Any), (Any, Any)]]
+  summon[ReverseOnto[(T1, T2), (T3, T4)] =:= Concat[Reverse[(T1, T2)], (T3, T4)]]
+
+def test2[Tup1 <: Tuple, Tup2 <: Tuple] =
+  summon[ReverseOnto[EmptyTuple, Tup1] =:= Tup1]
+  summon[ReverseOnto[Tup1, EmptyTuple] =:= Reverse[Tup1]]
+
+def test3[T1, T2, T3, T4](tup1: (T1, T2), tup2: (T3, T4)) =
+  summon[ReverseOnto[tup1.type, tup2.type] <:< (T2, T1, T3, T4)]
+  summon[ReverseOnto[tup1.type, tup2.type] =:= T2 *: T1 *: tup2.type]
diff --git a/tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala b/tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala
index a01c71724b0e..afa10ad6af6f 100644
--- a/tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala
+++ b/tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala
@@ -92,12 +92,10 @@ val experimentalDefinitionInLibrary = Set(
   "scala.quoted.Quotes.reflectModule.TermParamClauseMethods.hasErasedArgs",
 
   // New feature: reverse method on Tuple
-  "scala.Tuple.reverse",
-  "scala.Tuple$.Helpers",
-  "scala.Tuple$.Helpers$",
-  "scala.Tuple$.Helpers$.ReverseImpl",
-  "scala.Tuple$.Reverse",
-  "scala.runtime.Tuples$.reverse"
+  "scala.Tuple.reverse", // can be stabilized in 3.4
+  "scala.Tuple$.Reverse", // can be stabilized in 3.4
+  "scala.Tuple$.ReverseOnto", // can be stabilized in 3.4
+  "scala.runtime.Tuples$.reverse", // can be stabilized in 3.4
 )