Specialize char_traits for std::byte to fix from_msgpack (fixes #4756) (#4760)

* Specialize char_traits for std::byte to fix from_msgpack (fixes #4756)

Provide a char_traits<std::byte> specialization under __cpp_lib_byte
to allow parsing MessagePack data from containers of std::byte.

Signed-off-by: xuesongtap <tap91624@gmail.com>
Signed-off-by: yexiaochuan <tap91624@gmail.com>

* Fix comments for cstddef include and MessagePack tests

Signed-off-by: xuesongtap <tap91624@gmail.com>
Signed-off-by: yexiaochuan <tap91624@gmail.com>

* Fix include <cstddef> only when __cpp_lib_byte is defined and sufficient

Signed-off-by: yexiaochuan <tap91624@gmail.com>

* Fix clang-tidy warnings in MessagePack std::byte tests

Signed-off-by: yexiaochuan <tap91624@gmail.com>

* Fix handle return value in MessagePack tests

Signed-off-by: yexiaochuan <tap91624@gmail.com>

---------

Signed-off-by: xuesongtap <tap91624@gmail.com>
Signed-off-by: yexiaochuan <tap91624@gmail.com>

include/nlohmann/detail/meta/type_traits.hpp

@@ -13,7 +13,9 @@
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
-
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+    #include <cstddef> // byte
+#endif
 #include <nlohmann/detail/iterators/iterator_traits.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/call_std/begin.hpp>
@@ -239,6 +241,30 @@ struct char_traits<signed char> : std::char_traits<char>
     }
 };
 
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+template<>
+struct char_traits<std::byte> : std::char_traits<char>
+{
+    using char_type = std::byte;
+    using int_type = uint64_t;
+
+    static int_type to_int_type(char_type c) noexcept
+    {
+        return static_cast<int_type>(std::to_integer<unsigned char>(c));
+    }
+
+    static char_type to_char_type(int_type i) noexcept
+    {
+        return std::byte(static_cast<unsigned char>(i));
+    }
+
+    static constexpr int_type eof() noexcept
+    {
+        return static_cast<int_type>(std::char_traits<char>::eof());
+    }
+};
+#endif
+
 ///////////////////
 // is_ functions //
 ///////////////////

single_include/nlohmann/json.hpp

@@ -3367,7 +3367,9 @@ NLOHMANN_JSON_NAMESPACE_END
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
-
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+    #include <cstddef> // byte
+#endif
 // #include <nlohmann/detail/iterators/iterator_traits.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++
@@ -3776,6 +3778,30 @@ struct char_traits<signed char> : std::char_traits<char>
     }
 };
 
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+template<>
+struct char_traits<std::byte> : std::char_traits<char>
+{
+    using char_type = std::byte;
+    using int_type = uint64_t;
+
+    static int_type to_int_type(char_type c) noexcept
+    {
+        return static_cast<int_type>(std::to_integer<unsigned char>(c));
+    }
+
+    static char_type to_char_type(int_type i) noexcept
+    {
+        return std::byte(static_cast<unsigned char>(i));
+    }
+
+    static constexpr int_type eof() noexcept
+    {
+        return static_cast<int_type>(std::char_traits<char>::eof());
+    }
+};
+#endif
+
 ///////////////////
 // is_ functions //
 ///////////////////

tests/src/unit-msgpack.cpp

@@ -1880,3 +1880,80 @@ TEST_CASE("MessagePack roundtrips" * doctest::skip())
         }
     }
 }
+
+#ifdef JSON_HAS_CPP_17
+// Test suite for verifying MessagePack handling with std::byte input
+TEST_CASE("MessagePack with std::byte")
+{
+
+    SECTION("std::byte compatibility")
+    {
+        SECTION("vector roundtrip")
+        {
+            json original =
+            {
+                {"name", "test"},
+                {"value", 42},
+                {"array", {1, 2, 3}}
+            };
+
+            std::vector<uint8_t> temp = json::to_msgpack(original);
+            // Convert the uint8_t vector to std::byte vector
+            std::vector<std::byte> msgpack_data(temp.size());
+            for (size_t i = 0; i < temp.size(); ++i)
+            {
+                msgpack_data[i] = std::byte(temp[i]);
+            }
+            // Deserialize from std::byte vector back to JSON
+            json from_bytes;
+            CHECK_NOTHROW(from_bytes = json::from_msgpack(msgpack_data));
+
+            CHECK(from_bytes == original);
+        }
+
+        SECTION("empty vector")
+        {
+            const std::vector<std::byte> empty_data;
+            CHECK_THROWS_WITH_AS([&]() { 
+                [[maybe_unused]] auto result = json::from_msgpack(empty_data); 
+                return true; 
+            }(),
+            "[json.exception.parse_error.110] parse error at byte 1: syntax error while parsing MessagePack value: unexpected end of input",
+            json::parse_error&);
+        }
+
+        SECTION("comparison with workaround")
+        {
+            json original =
+            {
+                {"string", "hello"},
+                {"integer", 42},
+                {"float", 3.14},
+                {"boolean", true},
+                {"null", nullptr},
+                {"array", {1, 2, 3}},
+                {"object", {{"key", "value"}}}
+            };
+
+            std::vector<uint8_t> temp = json::to_msgpack(original);
+
+            std::vector<std::byte> msgpack_data(temp.size());
+            for (size_t i = 0; i < temp.size(); ++i)
+            {
+                msgpack_data[i] = std::byte(temp[i]);
+            }
+            // Attempt direct deserialization using std::byte input
+            json direct_result = json::from_msgpack(msgpack_data);
+
+            // Test the workaround approach: reinterpret as unsigned char* and use iterator range
+            const auto *const char_start = reinterpret_cast<unsigned char const*>(msgpack_data.data());
+            const auto *const char_end = char_start + msgpack_data.size();
+            json workaround_result = json::from_msgpack(char_start, char_end);
+
+            // Verify that the final deserialized JSON matches the original JSON
+            CHECK(direct_result == workaround_result);
+            CHECK(direct_result == original);
+        }
+    }
+}
+#endif