Use CMake 4.5's stable import std support in the module test (#6103)

* Use CMake 4.5's stable import std support in the module test

CMake 4.5 makes `import std` available to every target that scans for
C++ modules and drops the experimental CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
gate and the CXX_MODULE_STD property. The module test no longer needs the
version-specific opt-in token, so the ci_module_cpp20 jobs no longer have
to pin CMake 4.3.4 and now request CMake ^4.5.0.

The modules, CMake and README documentation now list the CMake, generator
and toolchain requirements for building the module and for `import std;`.

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

* Bump lukka/get-cmake from 4.4.5 to 4.5.0

get-cmake 4.4.5 does not know CMake 4.5.0, so the ci_module_cpp20 jobs
that request '^4.5.0' failed in the setup step. Bump every pin so all
jobs use the same action version.

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

* Install unzip in the Clang module job before get-cmake

The ^4.5.0 CMake/Ninja cache key is new, so get-cmake has to unzip the
Ninja download, and silkeh/clang:latest has no unzip.

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

* Disable C++ module scanning for the unit tests

With CMake 4.5, every target that scans for C++ modules gets a synthesized
std module target. For the C++20 and newer unit-test targets, this compiled
libstdc++'s std.cc with the -Werror test flags and failed to configure with
libc++ images that lack std.compat.cppm. The unit tests use no modules, so
turn scanning off; the module test is a separate project.

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

---------

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

.github/workflows/ubuntu.yml

@@ -25,7 +25,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -38,7 +38,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Check that Meson and pkg-config offer the CMake options
         run: make check_build_options
       - name: Install Meson
@@ -92,7 +92,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -123,7 +123,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -142,7 +142,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -168,7 +168,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -244,7 +244,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: CXX=g++-${{ matrix.compiler }} cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -262,7 +262,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -279,7 +279,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Disable filesystem support (std::filesystem is unusable) for clang 7 / 8 / 9 / 10
         run: echo "JSON_FORCED_GLOBAL_COMPILE_OPTIONS=-DJSON_HAS_FILESYSTEM=0;-DJSON_HAS_EXPERIMENTAL_FILESYSTEM=0" >> "$GITHUB_ENV"
         if: ${{ matrix.compiler == '7' || matrix.compiler == '8' || matrix.compiler == '9' || matrix.compiler == '10' }}
@@ -299,7 +299,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -319,7 +319,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build with libc++
@@ -346,7 +346,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -362,13 +362,16 @@ jobs:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         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@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+      # get-cmake extracts the Ninja zip with the system unzip, which the Clang
+      # image lacks; other Clang jobs get away with it by hitting the cache.
+      - name: Install unzip (Clang)
+        if: matrix.container == 'silkeh/clang:latest'
+        run: apt-get update ; apt-get install -y unzip
+      # The module test uses `import std;`, which needs CMake 4.5 or newer.
+      - name: Get CMake 4.5+ and ninja
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
         with:
-          cmakeVersion: 4.3.4
+          cmakeVersion: '^4.5.0'
       # 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.
@@ -392,7 +395,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -409,7 +412,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -423,7 +426,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -443,7 +446,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Run CMake
         run: cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=$EMSDK/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -GNinja
       - name: Build
@@ -484,7 +487,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Check that the job runs natively on arm64
         run: uname -m && test "$(uname -m)" = aarch64
       - name: Run CMake

.github/workflows/windows.yml

@@ -90,7 +90,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
       - name: Set extra CXX_FLAGS for latest std_version
         # /wd5285 silences C5285 emitted by the bundled third-party doctest.h, which
         # specializes std::tuple (newly diagnosed by the VS2026 v145 toolset)
@@ -212,13 +212,11 @@ jobs:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         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@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
+      # The module test uses `import std;`, which needs CMake 4.5 or newer.
+      - name: Get CMake 4.5+ and ninja
+        uses: lukka/get-cmake@16e8153a8b4918849d92aad131befe67a3705daa # v4.5.0
         with:
-          cmakeVersion: 4.3.4
+          cmakeVersion: '^4.5.0'
       # CMAKE_CXX_FLAGS replaces CMake's MSVC defaults (which include /EHsc), and
       # ci_module_cpp20 forwards it to the module build, so list /EHsc explicitly
       - name: Run CMake (Debug)

README.md

@@ -165,7 +165,8 @@ std::ifstream f("example.json");
 json data = json::parse(f);
 ```
 
-If using modules (enabled with `NLOHMANN_JSON_BUILD_MODULES`), this example becomes:
+If using modules (enabled with `NLOHMANN_JSON_BUILD_MODULES`; `import std;` needs CMake 4.5 or newer, see the
+[modules documentation](https://json.nlohmann.me/features/modules/)), this example becomes:
 ```cpp
 import std;
 import nlohmann.json;

docs/mkdocs/docs/features/modules.md

@@ -9,7 +9,19 @@ Importing the module instead of including the header reduces the compile time of
 ## Requirements
 The `nlohmann.json` module requires that the build system is configured to build and resolve modules when imported. Obviously, as modules were introduced in C++20, this feature can only be used in C++20 and subsequent versions.
 
-To enable building the `nlohmann.json` module (which is not done by default), the macro `NLOHMANN_JSON_BUILD_MODULES` must be passed to the build system.
+To enable building the `nlohmann.json` module (which is not done by default), set the CMake option
+[`NLOHMANN_JSON_BUILD_MODULES`](../integration/cmake.md#nlohmann_json_build_modules) and link the
+`nlohmann_json_modules` target.
+
+With CMake, building the module needs:
+
+- CMake 3.28 or newer, and
+- a generator that scans for modules: Ninja (1.11 or newer), Ninja Multi-Config, or Visual Studio 2022 or newer.
+
+The example below also uses `import std;`. CMake supports it without experimental opt-in since version 4.5, and then
+only with the Ninja generators and these toolchains: Clang 18.1.2 or newer (with libc++ or libstdc++), MSVC toolset 14.36
+(Visual Studio 17.6) or newer, and GCC 15 or newer (GCC 16 on macOS). With older CMake versions, `import std;` needs
+CMake's version-specific experimental opt-in.
 
 ## Example
 When using modules rather than headers, the previous example for creating a `json` object through a JSON file, would instead be:
@@ -67,7 +79,7 @@ and should not be used directly:
 
 ## Known issues
 
-C++20 modules support is exercised in CI against current GCC and Clang on Ubuntu, and the default MSVC toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md).
+C++20 modules support is exercised in CI with CMake 4.5 and `import std;` against current GCC and Clang on Ubuntu, and the default MSVC toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md).
 
 !!! info "Known compiler issues"
 

docs/mkdocs/docs/integration/cmake.md

@@ -269,6 +269,9 @@ target_link_libraries(myproject PRIVATE nlohmann_json_modules)
 target_compile_definitions(myproject PRIVATE NLOHMANN_JSON_BUILD_MODULES)
 ```
 
+If `main.cpp` also uses `import std;`, the project needs CMake 4.5 or newer and a Ninja generator; see
+[Modules](../features/modules.md#requirements) for the supported toolchains.
+
 ## See also
 
 - [Macros](../api/macros/index.md) - reference of all macros

tests/CMakeLists.txt

@@ -1,5 +1,12 @@
 cmake_minimum_required(VERSION 3.13...4.0)
 
+# The unit tests use no C++ modules. Without this, CMake 4.5 scans the C++20 and
+# newer test targets and builds a `std` module for them, compiling the standard
+# library's module sources with the test warning flags (which fails with
+# -Werror) or failing to configure when the libc++ module sources are missing.
+# The module test in tests/module_cpp20 is a separate project and unaffected.
+set(CMAKE_CXX_SCAN_FOR_MODULES OFF)
+
 option(JSON_Valgrind    "Execute test suite with Valgrind." OFF)
 option(JSON_FastTests   "Skip expensive/slow tests." OFF)
 option(JSON_TestSimdutf "Build the unit tests against the simdutf UTF-8 validation backend." OFF)

tests/module_cpp20/CMakeLists.txt

@@ -1,11 +1,6 @@
-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")
+# `import std;` (used by main.cpp) is supported without experimental opt-in
+# since CMake 4.5: it is available to every target that scans for C++ modules.
+cmake_minimum_required(VERSION 4.5)
 
 project(json_test CXX)
 
@@ -13,7 +8,6 @@ project(json_test CXX)
 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")
 

tests/module_cpp20/main.cpp

@@ -17,8 +17,8 @@ using namespace nlohmann::literals;
 // 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.
+// across GCC, Clang, and MSVC. This requires C++23 and CMake 4.5 or newer;
+// see CMakeLists.txt.
 int main()
 {
     // basic_json / json: parsing and value access