Overwork project infrastructure (#5218)

* 📝 overwork project infrastructure

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🐛 fix GCC16 issue

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🐛 fix GCC16 issue

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🐛 only build module for GCC

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🐛 fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 📝 fix documentation

Closes #5012: fix the error_handler_t::ignore wording

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 📝 fix documentation

Closes #4354: fix "Custom data source" example

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/module_cpp20/CMakeLists.txt

@@ -1,19 +1,28 @@
-cmake_minimum_required(VERSION 3.28)
+cmake_minimum_required(VERSION 3.30)
+
+# `import std;` (used by main.cpp) needs CMake's experimental C++ import-std
+# support. The accepted token is CMake-version-specific; this value matches the
+# CMake version pinned in the ci_module_cpp20 CI jobs (4.3.x). If that pin is
+# bumped, this token must be updated to match (configuration fails loudly
+# otherwise).
+set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "451f2fe2-a8a2-47c3-bc32-94786d8fc91b")
 
 project(json_test CXX)
 
+# import std; requires C++23 and the std library module
+set(CMAKE_CXX_STANDARD 23)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_MODULE_STD ON)
+
 set(NLOHMANN_JSON_BUILD_MODULES ON CACHE BOOL "Enable nlohmann.json module support")
 
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/../.. ${CMAKE_CURRENT_BINARY_DIR}/tests)
 
 add_executable(json_test main.cpp)
-target_link_libraries(json_test 
-    PRIVATE 
+target_link_libraries(json_test
+    PRIVATE
         nlohmann_json_modules
 )
 
-target_compile_definitions(json_test 
-    PRIVATE NLOHMANN_JSON_BUILD_MODULES
-)
-
-target_compile_features(json_test PUBLIC cxx_std_20)
+target_compile_features(json_test PRIVATE cxx_std_23)

tests/module_cpp20/main.cpp

@@ -6,9 +6,54 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
+import std;
 import nlohmann.json;
 
+using namespace nlohmann::literals;
+
+// Exercise the surface exported by the nlohmann.json module so that a missing
+// or broken export is caught at compile time.
+//
+// Standard-library facilities are pulled in via `import std;` rather than
+// textual `#include`s: mixing `import` with textual standard headers does not
+// compile under GCC's C++20 modules implementation, whereas `import std;` works
+// across GCC, Clang, and MSVC. This requires C++23 and CMake's (experimental)
+// import-std support; see CMakeLists.txt.
 int main()
 {
-    nlohmann::json j;
+    // basic_json / json: parsing and value access
+    nlohmann::json j = nlohmann::json::parse(R"({"a": 1, "list": [1, 2, 3]})");
+    const int a = j["a"].get<int>();
+
+    // json_pointer and operator""_json_pointer
+    nlohmann::json_pointer<std::string> ptr = "/list/2"_json_pointer;
+    const int last = j[ptr].get<int>();
+
+    // operator""_json literal
+    const nlohmann::json lit = R"([1, 2, 3])"_json;
+
+    // ordered_json
+    nlohmann::ordered_json oj;
+    oj["b"] = 2;
+    const int b = oj["b"].get<int>();
+
+    // ordered_map alias
+    nlohmann::ordered_map<std::string, int> m;
+    m["x"] = 1;
+
+    // adl_serializer (reference the exported template)
+    using serializer = nlohmann::adl_serializer<int, void>;
+    static_cast<void>(sizeof(serializer));
+
+    // to_string
+    const std::string dumped = nlohmann::to_string(j);
+
+    // operator<< (hidden friend, reached via ADL through the module)
+    std::ostringstream os;
+    os << j << oj << lit;
+
+    // use every result so the references cannot be optimized away
+    return (a == 1 && last == 3 && b == 2 && lit.size() == 3
+            && m.size() == 1 && !dumped.empty() && !os.str().empty())
+        ? 0 : 1;
 }