From 85dc04bdfa14c2f6cfc8fc71ea5968d11690ab93 Mon Sep 17 00:00:00 2001
From: Decel <8268812+Decel@users.noreply.github.com>
Date: Thu, 30 Mar 2023 21:58:00 +0200
Subject: [PATCH] Allow Tuple.Head and Tuple.Tail to accept any Tuple

---
 compiler/test/dotc/pos-test-pickling.blacklist | 2 ++
 library/src/scala/Tuple.scala                  | 4 ++--
 tests/{neg => pos}/i11982a.scala               | 8 ++++----
 tests/pos/i17186a.scala                        | 5 +++++
 tests/pos/i17186b.scala                        | 5 +++++
 5 files changed, 18 insertions(+), 6 deletions(-)
 rename tests/{neg => pos}/i11982a.scala (59%)
 create mode 100644 tests/pos/i17186a.scala
 create mode 100644 tests/pos/i17186b.scala

diff --git a/compiler/test/dotc/pos-test-pickling.blacklist b/compiler/test/dotc/pos-test-pickling.blacklist
index eb4b861eb324..b0da78f0a1eb 100644
--- a/compiler/test/dotc/pos-test-pickling.blacklist
+++ b/compiler/test/dotc/pos-test-pickling.blacklist
@@ -27,6 +27,8 @@ i9804.scala
 i13433.scala
 i16649-irrefutable.scala
 strict-pattern-bindings-3.0-migration.scala
+i17186b.scala
+i11982a.scala
 
 # Tree is huge and blows stack for printing Text
 i7034.scala
diff --git a/library/src/scala/Tuple.scala b/library/src/scala/Tuple.scala
index 6993b8202082..ae46f7fbf347 100644
--- a/library/src/scala/Tuple.scala
+++ b/library/src/scala/Tuple.scala
@@ -96,7 +96,7 @@ object Tuple {
   }
 
   /** Type of the head of a tuple */
-  type Head[X <: NonEmptyTuple] = X match {
+  type Head[X <: Tuple] = X match {
     case x *: _ => x
   }
 
@@ -108,7 +108,7 @@ object Tuple {
   }
 
   /** Type of the tail of a tuple */
-  type Tail[X <: NonEmptyTuple] <: Tuple = X match {
+  type Tail[X <: Tuple] <: Tuple = X match {
     case _ *: xs => xs
   }
 
diff --git a/tests/neg/i11982a.scala b/tests/pos/i11982a.scala
similarity index 59%
rename from tests/neg/i11982a.scala
rename to tests/pos/i11982a.scala
index 53126eaf3986..b3c68443ff04 100644
--- a/tests/neg/i11982a.scala
+++ b/tests/pos/i11982a.scala
@@ -6,9 +6,9 @@ object Unpair {
 
   def unpair[X <: Tuple2[?, ?]](
       using a: ValueOf[Tuple.Head[X]],
-            b: ValueOf[Tuple.Head[Tuple.Tail[X]]]  // error
-  ): Tuple2[Tuple.Head[X], Tuple.Head[Tuple.Tail[X]]] =  // error
+            b: ValueOf[Tuple.Head[Tuple.Tail[X]]]
+  ): Tuple2[Tuple.Head[X], Tuple.Head[Tuple.Tail[X]]] =
     type AA = Tuple.Head[X]
-    type BB = Tuple.Head[Tuple.Tail[X]]  // error
+    type BB = Tuple.Head[Tuple.Tail[X]]
     pair[AA, BB](using a, b)
-}
\ No newline at end of file
+}
diff --git a/tests/pos/i17186a.scala b/tests/pos/i17186a.scala
new file mode 100644
index 000000000000..7160ac001ba8
--- /dev/null
+++ b/tests/pos/i17186a.scala
@@ -0,0 +1,5 @@
+type second[X <: Tuple2[Any, Any]] = Tuple.Head[Tuple.Tail[X]]
+type middle[X <: Tuple3[Any, Any, Any]] = Tuple.Head[Tuple.Tail[X]]
+
+val a: Tuple.Head[Tuple.Tail[Tuple2[Int, String]]] = ???
+val b: Tuple.Head[Tuple.Tail[Tuple3[Int, String, Boolean]]] = ???
diff --git a/tests/pos/i17186b.scala b/tests/pos/i17186b.scala
new file mode 100644
index 000000000000..36efd647816c
--- /dev/null
+++ b/tests/pos/i17186b.scala
@@ -0,0 +1,5 @@
+type SecondOfTwo[X <: Tuple2[Any, Any]] = Tuple.Head[Tuple.Tail[X]]
+val a = implicitly[SecondOfTwo[Tuple2[Int, String]] =:= String]
+
+type LastOfThree[X <: Tuple3[Any, Any, Any]] = Tuple.Tail[Tuple.Tail[X]]
+val b = implicitly[LastOfThree[Tuple3[Int, String, Boolean]] =:= Tuple1[Boolean]]