Add proper C++20 module support (#4799)

src/modules/CMakeLists.txt

@@ -0,0 +1,27 @@
+cmake_minimum_required(VERSION 3.28)
+
+add_library(nlohmann_json_modules)
+
+set(NLOHMANN_JSON_MODULES
+    json.cppm
+)
+
+if(NOT COMMAND configure_cpp_module_target)
+    function(configure_cpp_module_target target)
+        target_sources(${target} PUBLIC FILE_SET CXX_MODULES FILES ${NLOHMANN_JSON_MODULES})
+    endfunction()
+endif()
+
+configure_cpp_module_target(nlohmann_json_modules)
+
+target_link_libraries(nlohmann_json_modules
+    PUBLIC
+    nlohmann_json::nlohmann_json
+)
+
+target_include_directories(nlohmann_json_modules
+    PRIVATE
+    ${PROJECT_SOURCE_DIR}/include
+)
+
+target_compile_features(nlohmann_json_modules PUBLIC cxx_std_20)

src/modules/json.cppm

@@ -0,0 +1,22 @@
+module;
+
+#include <nlohmann/json.hpp>
+
+export module nlohmann.json;
+
+export namespace nlohmann {
+    template <typename T = void, typename SFINAE = void>
+    using adl_serializer = ::nlohmann::adl_serializer<T, SFINAE>;
+
+    using basic_json = ::nlohmann::basic_json<>;
+
+    using json = ::nlohmann::json;
+
+    template <typename RefStringType>
+    using json_pointer = ::nlohmann::json_pointer<RefStringType>;
+
+    using ::nlohmann::ordered_json;
+
+    template <class Key, class T, class IgnoredLess, class Allocator>
+    using ordered_map = ::nlohmann::ordered_map<Key, T, IgnoredLess, Allocator>;
+}  // namespace nlohmann