From 93e20c9afa13197984f3033659d1c724cf2ab769 Mon Sep 17 00:00:00 2001
From: Sergiu Deitsch <sergiud@users.noreply.github.com>
Date: Wed, 23 Apr 2025 18:36:41 +0200
Subject: [PATCH] Provide fallback for missing `char8_t` support (#4736)

---
 .../nlohmann/detail/conversions/from_json.hpp |  5 +++-
 .../nlohmann/detail/conversions/to_json.hpp   | 21 ++++++++++-----
 single_include/nlohmann/json.hpp              | 26 +++++++++++++------
 tests/src/unit-deserialization.cpp            |  5 ++--
 4 files changed, 39 insertions(+), 18 deletions(-)

diff --git a/include/nlohmann/detail/conversions/from_json.hpp b/include/nlohmann/detail/conversions/from_json.hpp
index d647d742..e161a428 100644
--- a/include/nlohmann/detail/conversions/from_json.hpp
+++ b/include/nlohmann/detail/conversions/from_json.hpp
@@ -540,7 +540,10 @@ inline void from_json(const BasicJsonType& j, std_fs::path& p)
         JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
     const auto& s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
-#ifdef JSON_HAS_CPP_20
+    // Checking for C++20 standard or later can be insufficient in case the
+    // library support for char8_t is either incomplete or was disabled
+    // altogether. Use the __cpp_lib_char8_t feature test instead.
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
     p = std_fs::path(std::u8string_view(reinterpret_cast<const char8_t*>(s.data()), s.size()));
 #else
     p = std_fs::u8path(s); // accepts UTF-8 encoded std::string in C++17, deprecated in C++20
diff --git a/include/nlohmann/detail/conversions/to_json.hpp b/include/nlohmann/detail/conversions/to_json.hpp
index ead45665..b17e8af4 100644
--- a/include/nlohmann/detail/conversions/to_json.hpp
+++ b/include/nlohmann/detail/conversions/to_json.hpp
@@ -15,7 +15,8 @@
 
 #include <algorithm> // copy
 #include <iterator> // begin, end
-#include <string> // string
+#include <memory> // allocator_traits
+#include <string> //  basic_string, char_traits
 #include <tuple> // tuple, get
 #include <type_traits> // is_same, is_constructible, is_floating_point, is_enum, underlying_type
 #include <utility> // move, forward, declval, pair
@@ -440,15 +441,21 @@ inline void to_json(BasicJsonType& j, const T& t)
 }
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+#if defined(__cpp_lib_char8_t)
+template<typename BasicJsonType, typename Tr, typename Allocator>
+inline void to_json(BasicJsonType& j, const std::basic_string<char8_t, Tr, Allocator>& s)
+{
+    using OtherAllocator = typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
+    j = std::basic_string<char, std::char_traits<char>, OtherAllocator>(s.begin(), s.end(), s.get_allocator());
+}
+#endif
+
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, const std_fs::path& p)
 {
-#ifdef JSON_HAS_CPP_20
-    const std::u8string s = p.u8string();
-    j = std::string(s.begin(), s.end());
-#else
-    j = p.u8string(); // returns std::string in C++17
-#endif
+    // Returns either a std::string or a std::u8string depending whether library
+    // support for char8_t is enabled.
+    j = p.u8string();
 }
 #endif
 
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 82d69f7c..be3493ef 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -5325,7 +5325,10 @@ inline void from_json(const BasicJsonType& j, std_fs::path& p)
         JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
     const auto& s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
-#ifdef JSON_HAS_CPP_20
+    // Checking for C++20 standard or later can be insufficient in case the
+    // library support for char8_t is either incomplete or was disabled
+    // altogether. Use the __cpp_lib_char8_t feature test instead.
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
     p = std_fs::path(std::u8string_view(reinterpret_cast<const char8_t*>(s.data()), s.size()));
 #else
     p = std_fs::u8path(s); // accepts UTF-8 encoded std::string in C++17, deprecated in C++20
@@ -5380,7 +5383,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 #include <algorithm> // copy
 #include <iterator> // begin, end
-#include <string> // string
+#include <memory> // allocator_traits
+#include <string> //  basic_string, char_traits
 #include <tuple> // tuple, get
 #include <type_traits> // is_same, is_constructible, is_floating_point, is_enum, underlying_type
 #include <utility> // move, forward, declval, pair
@@ -6087,15 +6091,21 @@ inline void to_json(BasicJsonType& j, const T& t)
 }
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+#if defined(__cpp_lib_char8_t)
+template<typename BasicJsonType, typename Tr, typename Allocator>
+inline void to_json(BasicJsonType& j, const std::basic_string<char8_t, Tr, Allocator>& s)
+{
+    using OtherAllocator = typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
+    j = std::basic_string<char, std::char_traits<char>, OtherAllocator>(s.begin(), s.end(), s.get_allocator());
+}
+#endif
+
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, const std_fs::path& p)
 {
-#ifdef JSON_HAS_CPP_20
-    const std::u8string s = p.u8string();
-    j = std::string(s.begin(), s.end());
-#else
-    j = p.u8string(); // returns std::string in C++17
-#endif
+    // Returns either a std::string or a std::u8string depending whether library
+    // support for char8_t is enabled.
+    j = p.u8string();
 }
 #endif