From 7731a91d75e2cb871099a5d57df4e84050d8462e Mon Sep 17 00:00:00 2001
From: Neil MacIntosh <neilmac@microsoft.com>
Date: Wed, 31 May 2017 18:42:06 -0700
Subject: [PATCH] Use std::byte when available. (#519)

* Use std::byte implementation when available with MSVC.

* Rollback the /std:c++latest flag used for testing.

* Review feedback.
---
 include/gsl/gsl_byte | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/include/gsl/gsl_byte b/include/gsl/gsl_byte
index 91a28ed..070e32b 100644
--- a/include/gsl/gsl_byte
+++ b/include/gsl/gsl_byte
@@ -37,10 +37,44 @@
 #pragma push_macro("noexcept")
 #define noexcept /*noexcept*/
 #endif           // _MSC_VER <= 1800
+
+// this tests if we are under MSVC and the standard lib has std::byte and it is enabled
+#if _MSC_VER >= 1911 && (!defined(_HAS_STD_BYTE) || _HAS_STD_BYTE) 
+
+#define GSL_USE_STD_BYTE 1
+
+#else // _MSC_VER >= 1911 && (!defined(_HAS_STD_BYTE) || _HAS_STD_BYTE) 
+
+#define GSL_USE_STD_BYTE 0
+
+#endif // _MSC_VER >= 1911 && (!defined(_HAS_STD_BYTE) || _HAS_STD_BYTE) 
+
+#else // _MSC_VER
+
+// this tests if we are under GCC or Clang with enough -std:c++1z power to get us std::byte
+#if defined(__cplusplus) && (__cplusplus > 201703L)
+
+#define GSL_USE_STD_BYTE 1
+
+#else // defined(__cplusplus) && (__cplusplus > 201703L)
+
+#define GSL_USE_STD_BYTE 0
+
+#endif //defined(__cplusplus) && (__cplusplus > 201703L)
+
 #endif           // _MSC_VER
 
 namespace gsl
 {
+#if GSL_USE_STD_BYTE
+
+#include <cstddef>
+
+using std::byte;
+using std::to_integer;
+
+#else // GSL_USE_STD_BYTE
+
 // This is a simple definition for now that allows
 // use of byte within span<> to be standards-compliant
 enum class byte : unsigned char
@@ -109,6 +143,8 @@ inline constexpr IntegerType to_integer(byte b) noexcept
     return static_cast<IntegerType>(b);
 }
 
+#endif // GSL_USE_STD_BYTE
+
 template <bool E, typename T>
 inline constexpr byte to_byte_impl(T t) noexcept
 {