From 85ad05396a1e3ef8e7468974a6ae653b84711f2a Mon Sep 17 00:00:00 2001
From: MattStephanson <68978048+MattStephanson@users.noreply.github.com>
Date: Thu, 14 Jan 2021 18:28:03 -0800
Subject: [PATCH] <algorithm>: fill should accept volatile byte pointers
 (#1557)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>
---
 stl/inc/xutility                                 | 5 +++--
 tests/std/tests/VSO_0180469_fill_family/test.cpp | 3 +++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/stl/inc/xutility b/stl/inc/xutility
index 653646d54..f484b23b1 100644
--- a/stl/inc/xutility
+++ b/stl/inc/xutility
@@ -4361,11 +4361,12 @@ struct _Is_character_or_byte_or_bool<byte> : true_type {};
 template <>
 struct _Is_character_or_byte_or_bool<bool> : true_type {};
 
-// _Fill_memset_is_safe determines if _FwdIt and _Ty are eligible for memset optimization in fill
+// _Fill_memset_is_safe determines if _FwdIt and _Ty are eligible for memset optimization in fill.
+// Need to explicitly test for volatile because _Unwrap_enum_t discards qualifiers.
 template <class _FwdIt, class _Ty, bool = is_pointer_v<_FwdIt>>
 _INLINE_VAR constexpr bool _Fill_memset_is_safe = conjunction_v<is_scalar<_Ty>,
     _Is_character_or_byte_or_bool<_Unwrap_enum_t<remove_reference_t<_Iter_ref_t<_FwdIt>>>>,
-    is_assignable<_Iter_ref_t<_FwdIt>, const _Ty&>>;
+    negation<is_volatile<remove_reference_t<_Iter_ref_t<_FwdIt>>>>, is_assignable<_Iter_ref_t<_FwdIt>, const _Ty&>>;
 
 template <class _FwdIt, class _Ty>
 _INLINE_VAR constexpr bool _Fill_memset_is_safe<_FwdIt, _Ty, false> = false;
diff --git a/tests/std/tests/VSO_0180469_fill_family/test.cpp b/tests/std/tests/VSO_0180469_fill_family/test.cpp
index c932d75f0..6c3bef212 100644
--- a/tests/std/tests/VSO_0180469_fill_family/test.cpp
+++ b/tests/std/tests/VSO_0180469_fill_family/test.cpp
@@ -134,6 +134,9 @@ int main() {
     test_fill<char, int>();
 
     test_fill<volatile char, char>(); // Test GH-1183
+#ifdef __cpp_lib_byte
+    test_fill<volatile byte, byte>(); // Test GH-1556
+#endif // __cpp_lib_byte
 
     test_uninitialized_fill(
         [](count_copies* buff, size_t n, const count_copies& src) { uninitialized_fill(buff, buff + n, src); });