🐛 fix build

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

.github/workflows/ubuntu.yml

@@ -247,18 +247,28 @@ jobs:
   ci_module_cpp20:
     strategy:
       matrix:
-        # Only Clang for now: GCC's C++20 modules support (as of GCC 16.1) cannot
-        # compile non-trivial use of the imported module (redefinition / missing
-        # include errors in libstdc++).
-        container: ['silkeh/clang:latest']
+        container: ['gcc:latest', 'silkeh/clang:latest']
     runs-on: ubuntu-latest
     container: ${{ matrix.container }}
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
           persist-credentials: false
-      - name: Get latest CMake and ninja
+      # The module test uses `import std;`, which needs CMake's experimental
+      # import-std support. Its opt-in token is CMake-version-specific, so pin
+      # CMake to the version whose token is set in tests/module_cpp20/CMakeLists.txt.
+      - name: Get pinned CMake and ninja
         uses: lukka/get-cmake@f5b8fbb4d77cec1acc5a5f9f0df4beffaf5d98d9 # v4.3.4
+        with:
+          cmakeVersion: 4.3.4
+      # Clang: the std library module is provided by libc++ (the image's libstdc++
+      # ships none), and the image's libc++ module manifest has a broken relative
+      # path — repoint it at the real module sources.
+      - name: Use libc++ and fix its module manifest path (Clang)
+        if: matrix.container == 'silkeh/clang:latest'
+        run: |
+          echo "CXXFLAGS=-stdlib=libc++" >> "$GITHUB_ENV"
+          mkdir -p /usr/lib/share && ln -sf /usr/lib/llvm-*/share/libc++ /usr/lib/share/libc++
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build

.github/workflows/windows.yml

@@ -127,6 +127,13 @@ jobs:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
           persist-credentials: false
+      # The module test uses `import std;`, which needs CMake's experimental
+      # import-std support. Its opt-in token is CMake-version-specific, so pin
+      # CMake to the version whose token is set in tests/module_cpp20/CMakeLists.txt.
+      - name: Get pinned CMake and ninja
+        uses: lukka/get-cmake@f5b8fbb4d77cec1acc5a5f9f0df4beffaf5d98d9 # v4.3.4
+        with:
+          cmakeVersion: 4.3.4
       - name: Run CMake (Debug)
         run: cmake -S . -B build -G "Visual Studio 17 2022" -DJSON_CI=ON -DCMAKE_CXX_FLAGS="/permissive- /std:c++latest /utf-8 /W4 /WX"
       - name: Build

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