From 9a95027b70ab1349ab7338568e10fa3b292bedcc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vojt=C4=9Bch=20Michal?= <vojta.michall@email.cz>
Date: Fri, 21 Nov 2025 15:51:55 +0100
Subject: [PATCH 1/4] Add tests

---
 .../test.compile.pass.cpp                            | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp b/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
index 53c28d0c0e2..0362385173f 100644
--- a/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
+++ b/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
@@ -122,11 +122,23 @@ static_assert(enable_nonlocking_formatter_optimization<nullptr_t>);
 static_assert(enable_nonlocking_formatter_optimization<void*>);
 static_assert(enable_nonlocking_formatter_optimization<const void*>);
 static_assert(enable_nonlocking_formatter_optimization<pair<int, int>>);
+static_assert(enable_nonlocking_formatter_optimization<pair<int&, int>>);
+static_assert(enable_nonlocking_formatter_optimization<pair<int, int&>>);
 static_assert(!enable_nonlocking_formatter_optimization<pair<unoptimized, int>>);
+static_assert(!enable_nonlocking_formatter_optimization<pair<unoptimized&, int>>);
+static_assert(!enable_nonlocking_formatter_optimization<pair<unoptimized, int&>>);
 static_assert(!enable_nonlocking_formatter_optimization<pair<int, unoptimized>>);
+static_assert(!enable_nonlocking_formatter_optimization<pair<int&, unoptimized>>);
+static_assert(!enable_nonlocking_formatter_optimization<pair<int, unoptimized&>>);
 static_assert(enable_nonlocking_formatter_optimization<tuple<int, int>>);
+static_assert(enable_nonlocking_formatter_optimization<tuple<int&, int>>);
+static_assert(enable_nonlocking_formatter_optimization<tuple<int, int&>>);
 static_assert(!enable_nonlocking_formatter_optimization<tuple<unoptimized, int>>);
+static_assert(!enable_nonlocking_formatter_optimization<tuple<unoptimized&, int>>);
+static_assert(!enable_nonlocking_formatter_optimization<tuple<unoptimized, int&>>);
 static_assert(!enable_nonlocking_formatter_optimization<tuple<int, unoptimized>>);
+static_assert(!enable_nonlocking_formatter_optimization<tuple<int&, unoptimized>>);
+static_assert(!enable_nonlocking_formatter_optimization<tuple<int, unoptimized&>>);
 
 // Validate that various ranges are unoptimized
 static_assert(!enable_nonlocking_formatter_optimization<list<int>>);

From c288f741df2bc107dc4caca1938918f69e180769 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vojt=C4=9Bch=20Michal?= <vojta.michall@email.cz>
Date: Fri, 21 Nov 2025 15:53:04 +0100
Subject: [PATCH 2/4] Apply remove_cv_ref_t in specialization of
 enable_nonlocking_formatter_optimization for pair and tuple

---
 stl/inc/__msvc_formatter.hpp | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/stl/inc/__msvc_formatter.hpp b/stl/inc/__msvc_formatter.hpp
index d1355eb20fc..8fa4c3017e4 100644
--- a/stl/inc/__msvc_formatter.hpp
+++ b/stl/inc/__msvc_formatter.hpp
@@ -401,11 +401,12 @@ constexpr bool enable_nonlocking_formatter_optimization<basic_string_view<_CharT
 
 template <class _Ty1, class _Ty2>
 constexpr bool enable_nonlocking_formatter_optimization<pair<_Ty1, _Ty2>> =
-    enable_nonlocking_formatter_optimization<_Ty1> && enable_nonlocking_formatter_optimization<_Ty2>;
+    enable_nonlocking_formatter_optimization<remove_cvref_t<_Ty1>>
+    && enable_nonlocking_formatter_optimization<remove_cvref_t<_Ty2>>;
 
 template <class... _Ts>
 constexpr bool enable_nonlocking_formatter_optimization<tuple<_Ts...>> =
-    (enable_nonlocking_formatter_optimization<_Ts> && ...);
+    (enable_nonlocking_formatter_optimization<remove_cvref_t<_Ts>> && ...);
 
 template <class _CharT>
 struct _Fill_align_and_width_specs {

From 68b801e675183c0fe2b03d8cfaebac8613b7ea2a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vojt=C4=9Bch=20Michal?= <vojta.michall@email.cz>
Date: Fri, 21 Nov 2025 15:57:26 +0100
Subject: [PATCH 3/4] Mention LWG-4399 in the test suite

---
 .../tests/P3107R5_enabled_specializations/test.compile.pass.cpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp b/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
index 0362385173f..aeec890bbda 100644
--- a/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
+++ b/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
@@ -14,6 +14,8 @@
 #include <utility>
 #include <vector>
 
+// Extended to test LWG-4399 "enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t"
+
 using namespace std;
 
 struct unoptimized {};

From e3a089138a8f3b832b9cb21490d8bb9accd56f06 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Sun, 23 Nov 2025 14:18:39 -0800
Subject: [PATCH 4/4] Extract helpers to also test `const T` and `const T&`.

---
 .../test.compile.pass.cpp                     | 43 ++++++++++---------
 1 file changed, 23 insertions(+), 20 deletions(-)

diff --git a/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp b/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
index aeec890bbda..2eed1af8c39 100644
--- a/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
+++ b/tests/std/tests/P3107R5_enabled_specializations/test.compile.pass.cpp
@@ -14,8 +14,6 @@
 #include <utility>
 #include <vector>
 
-// Extended to test LWG-4399 "enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t"
-
 using namespace std;
 
 struct unoptimized {};
@@ -123,24 +121,29 @@ static_assert(enable_nonlocking_formatter_optimization<long double>);
 static_assert(enable_nonlocking_formatter_optimization<nullptr_t>);
 static_assert(enable_nonlocking_formatter_optimization<void*>);
 static_assert(enable_nonlocking_formatter_optimization<const void*>);
-static_assert(enable_nonlocking_formatter_optimization<pair<int, int>>);
-static_assert(enable_nonlocking_formatter_optimization<pair<int&, int>>);
-static_assert(enable_nonlocking_formatter_optimization<pair<int, int&>>);
-static_assert(!enable_nonlocking_formatter_optimization<pair<unoptimized, int>>);
-static_assert(!enable_nonlocking_formatter_optimization<pair<unoptimized&, int>>);
-static_assert(!enable_nonlocking_formatter_optimization<pair<unoptimized, int&>>);
-static_assert(!enable_nonlocking_formatter_optimization<pair<int, unoptimized>>);
-static_assert(!enable_nonlocking_formatter_optimization<pair<int&, unoptimized>>);
-static_assert(!enable_nonlocking_formatter_optimization<pair<int, unoptimized&>>);
-static_assert(enable_nonlocking_formatter_optimization<tuple<int, int>>);
-static_assert(enable_nonlocking_formatter_optimization<tuple<int&, int>>);
-static_assert(enable_nonlocking_formatter_optimization<tuple<int, int&>>);
-static_assert(!enable_nonlocking_formatter_optimization<tuple<unoptimized, int>>);
-static_assert(!enable_nonlocking_formatter_optimization<tuple<unoptimized&, int>>);
-static_assert(!enable_nonlocking_formatter_optimization<tuple<unoptimized, int&>>);
-static_assert(!enable_nonlocking_formatter_optimization<tuple<int, unoptimized>>);
-static_assert(!enable_nonlocking_formatter_optimization<tuple<int&, unoptimized>>);
-static_assert(!enable_nonlocking_formatter_optimization<tuple<int, unoptimized&>>);
+
+// Extended to test LWG-4399 "enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t"
+template <template <class, class> class PairOrTuple, class T, class U, bool Expected>
+constexpr void test_lwg_4399_impl() {
+    static_assert(enable_nonlocking_formatter_optimization<PairOrTuple<T, U>> == Expected);
+    static_assert(enable_nonlocking_formatter_optimization<PairOrTuple<T&, U>> == Expected);
+    static_assert(enable_nonlocking_formatter_optimization<PairOrTuple<T, U&>> == Expected);
+    static_assert(enable_nonlocking_formatter_optimization<PairOrTuple<const T, U>> == Expected);
+    static_assert(enable_nonlocking_formatter_optimization<PairOrTuple<T, const U>> == Expected);
+    static_assert(enable_nonlocking_formatter_optimization<PairOrTuple<const T&, U>> == Expected);
+    static_assert(enable_nonlocking_formatter_optimization<PairOrTuple<T, const U&>> == Expected);
+}
+
+template <class T, class U, bool Expected>
+constexpr bool test_lwg_4399() {
+    test_lwg_4399_impl<pair, T, U, Expected>();
+    test_lwg_4399_impl<tuple, T, U, Expected>();
+    return true;
+}
+
+static_assert(test_lwg_4399<int, int, true>());
+static_assert(test_lwg_4399<unoptimized, int, false>());
+static_assert(test_lwg_4399<int, unoptimized, false>());
 
 // Validate that various ranges are unoptimized
 static_assert(!enable_nonlocking_formatter_optimization<list<int>>);