gnu: Add nlohmann-json-no-char8-t.

* gnu/packages/patches/nlohmann_json_fix_char8_t.patch: New file.
* gnu/local.mk: (dist_patch_DATA): Register it.
* gnu/packages/cpp.scm (nlohmann-json-no-char8-t): New variable.

Change-Id: I8b47d345cd82718d1a2bd40009c480aca7c2c5b3
Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>

3 files changed, 185 insertions, 0 deletions

diff --git a/gnu/local.mk b/gnu/local.mk
index e48b5bf29c0..289d8461cd9 100644
--- a/gnu/local.mk
+++ b/gnu/local.mk
@@ -1993,6 +1993,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/nhc98-c-update.patch                     \
   %D%/packages/patches/nheko-0-12-1-fix-rendering-replies.patch \
   %D%/packages/patches/nix-dont-build-html-doc.diff             \
+  %D%/packages/patches/nlohmann_json_fix_char8_t.patch          \
   %D%/packages/patches/nfs4-acl-tools-0.3.7-fixpaths.patch      \
   %D%/packages/patches/neovim-tree-sitter-grammar-path.patch    \
   %D%/packages/patches/network-manager-plugin-ownership.patch   \
diff --git a/gnu/packages/cpp.scm b/gnu/packages/cpp.scm
index 40d16fc5dc7..d2a86bfc97a 100644
--- a/gnu/packages/cpp.scm
+++ b/gnu/packages/cpp.scm
@@ -1434,6 +1434,9 @@ data transfer object.")
 (define-public nlohmann-json
   (package
     (name "nlohmann-json")
+    ;; XXX: Merge related package `nlohmann-json-no-char8-t' back into here when
+    ;; updating from 3.12.0, as the unique patch it applies will already be
+    ;; incorporated in.
     (version "3.12.0")
     (home-page "https://github.com/nlohmann/json")
     (source
@@ -1507,6 +1510,46 @@ intuitive syntax and trivial integration.")
 (define-deprecated-package json-modern-cxx
   nlohmann-json)
 
+(define-public nlohmann-json-no-char8-t
+  (package
+    ;; XXX: Version 3.12.0 does not work without char8_t support.
+    ;; OpenRCT2 compiles with -fno-char8_t, and thus requires this patch.
+    ;; See https://github.com/nlohmann/json/pull/4736
+    (inherit nlohmann-json)
+    (name "nlohmann-json-no-char8-t")
+    (version "3.12.0")
+    (source
+     (origin
+       (method git-fetch)
+       (uri (git-reference (url "https://github.com/nlohmann/json")
+                           (commit (string-append "v" version))))
+       (sha256
+        (base32 "09nqq56ighr3lghhn3fs399lkllghz717j0xyp87x0giw86ayh3h"))
+       (file-name (git-file-name name version))
+       (modules '((guix build utils)))
+       (snippet
+        #~(begin
+            ;; Delete bundled software.  Preserve doctest_compatibility.h, which
+            ;; is a wrapper library added by this package.
+            (install-file "./tests/thirdparty/doctest/doctest_compatibility.h"
+                          "/tmp")
+            (delete-file-recursively "./tests/thirdparty")
+            (install-file "/tmp/doctest_compatibility.h"
+                          "./tests/thirdparty/doctest")
+
+            ;; Adjust for the unbundled fifo_map and doctest.
+            (substitute* (find-files "./tests/" "\\.h(pp)?")
+              (("#include \"doctest\\.h\"") "#include <doctest/doctest.h>")
+              (("#include <doctest\\.h>") "#include <doctest/doctest.h>"))
+            (with-directory-excursion "tests/src"
+              (let ((files (find-files "." "\\.cpp$")))
+                (substitute* files
+                  (("#include ?\"(fifo_map.hpp)\"" all fifo-map-hpp)
+                   (string-append
+                    "#include <fifo_map/" fifo-map-hpp ">")))))))
+       (patches
+        (search-patches "nlohmann_json_fix_char8_t.patch"))))))
+
 (define-public jthread
   (let ((commit "0fa8d394254886c555d6faccd0a3de819b7d47f8")
         (revision "0"))
diff --git a/gnu/packages/patches/nlohmann_json_fix_char8_t.patch b/gnu/packages/patches/nlohmann_json_fix_char8_t.patch
new file mode 100644
index 00000000000..014a328dfe9
--- /dev/null
+++ b/gnu/packages/patches/nlohmann_json_fix_char8_t.patch
@@ -0,0 +1,141 @@
+From 756ca22ec5b0d89b5d107b4c30891d1293650c87 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 797f714dfae1dcc7fd8d2435ae7015023da76b97..3a24a6f4d26c0162531d8100a99a05291d8a2090 100644
+--- a/include/nlohmann/detail/conversions/from_json.hpp
++++ b/include/nlohmann/detail/conversions/from_json.hpp
+@@ -539,7 +539,10 @@ 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 f8413850d5d7f8dd4bd6d2f174959a8ddd5c233a..8b910dd1615b710308392eed4999d3b7533a26e7 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 13b07c0fbc87e5c24127922fb2ef45fc3ea2bff5..93e5983cf1b1cdbe0452d89dcfa487c7b18dd4df 100644
+--- a/single_include/nlohmann/json.hpp
++++ b/single_include/nlohmann/json.hpp
+@@ -5324,7 +5324,10 @@ 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
+@@ -5379,7 +5382,8 @@ 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
+@@ -6086,15 +6090,21 @@ 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/tests/src/unit-deserialization.cpp b/tests/src/unit-deserialization.cpp
+index 84a970a183e4392166a8fe9bf76b582a53b76dd5..5c450c23d3042991cfc8959d4632208cfb21f400 100644
+--- a/tests/src/unit-deserialization.cpp
++++ b/tests/src/unit-deserialization.cpp
+@@ -1134,9 +1134,10 @@ TEST_CASE("deserialization")
+     }
+ }
+ 
+-// select the types to test - char8_t is only available in C++20
++// select the types to test - char8_t is only available since C++20 if and only
++// if __cpp_char8_t is defined.
+ #define TYPE_LIST(...) __VA_ARGS__
+-#ifdef JSON_HAS_CPP_20
++#if defined(__cpp_char8_t) && (__cpp_char8_t >= 201811L)
+     #define ASCII_TYPES TYPE_LIST(char, wchar_t, char16_t, char32_t, char8_t)
+ #else
+     #define ASCII_TYPES TYPE_LIST(char, wchar_t, char16_t, char32_t)