From 4983ebb94f8abc9cdfa4b46ff31910c3f03b71b3 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                  | 9 +++++++--
 tests/{neg => pos}/i11982a.scala               | 0
 tests/pos/i17186.scala                         | 5 +++++
 4 files changed, 14 insertions(+), 2 deletions(-)
 rename tests/{neg => pos}/i11982a.scala (100%)
 create mode 100644 tests/pos/i17186.scala

diff --git a/compiler/test/dotc/pos-test-pickling.blacklist b/compiler/test/dotc/pos-test-pickling.blacklist
index d1dd83f36ff7..97663bbcc09b 100644
--- a/compiler/test/dotc/pos-test-pickling.blacklist
+++ b/compiler/test/dotc/pos-test-pickling.blacklist
@@ -21,6 +21,8 @@ i15181.scala
 i15922.scala
 t5031_2.scala
 i16997.scala
+i17186.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 fa72e320b560..1157cec919fa 100644
--- a/library/src/scala/Tuple.scala
+++ b/library/src/scala/Tuple.scala
@@ -89,7 +89,11 @@ object Tuple {
   }
 
   /** Type of the head of a tuple */
-  type Head[X <: NonEmptyTuple] = X match {
+  // Only bounded by `<: Tuple` not `<: NonEmptyTuple`
+  // even though it only matches non-empty tuples.
+  // This allows an irreducible type like `Tuple.Head[Tuple.Tail[X]]`
+  // to avoid running into type bounds violation checks.
+  type Head[X <: Tuple] = X match {
     case x *: _ => x
   }
 
@@ -101,7 +105,8 @@ object Tuple {
   }
 
   /** Type of the tail of a tuple */
-  type Tail[X <: NonEmptyTuple] <: Tuple = X match {
+  // See Head for why not `<: NonEmptyTuple`
+  type Tail[X <: Tuple] <: Tuple = X match {
     case _ *: xs => xs
   }
 
diff --git a/tests/neg/i11982a.scala b/tests/pos/i11982a.scala
similarity index 100%
rename from tests/neg/i11982a.scala
rename to tests/pos/i11982a.scala
diff --git a/tests/pos/i17186.scala b/tests/pos/i17186.scala
new file mode 100644
index 000000000000..36efd647816c
--- /dev/null
+++ b/tests/pos/i17186.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]]