🐛 fix build

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

tests/module_cpp20/CMakeLists.txt

@@ -1,7 +1,20 @@
-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)
@@ -12,4 +25,4 @@ target_link_libraries(json_test
         nlohmann_json_modules
 )
 
-target_compile_features(json_test PRIVATE cxx_std_20)
+target_compile_features(json_test PRIVATE cxx_std_23)

tests/module_cpp20/main.cpp

@@ -6,22 +6,19 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
+import std;
 import nlohmann.json;
 
-#include <sstream>
-#include <string>
-
 using namespace nlohmann::literals;
 
-// This test exercises the surface exported by the nlohmann.json module so that
-// a missing or broken export is caught at compile time.
+// Exercise the surface exported by the nlohmann.json module so that a missing
+// or broken export is caught at compile time.
 //
-// NOTE: It is only built with Clang. GCC's C++20 modules implementation (as of
-// GCC 16.1) cannot compile non-trivial use of the imported module: any standard
-// library facility reached through the module triggers redefinition or missing
-// include errors in libstdc++ (and using `import` together with textual
-// `#include`s of standard headers fails as well). The ci_module_cpp20 job is
-// therefore restricted to Clang until GCC's modules support matures.
+// 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()
 {
     // basic_json / json: parsing and value access