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>

.github/workflows/ubuntu.yml

@@ -362,13 +362,11 @@ jobs:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
-      # The module test uses `import std;`, which needs CMake's experimental
+      # The module test uses `import std;`, which needs CMake 4.5 or newer.
-      # import-std support. Its opt-in token is CMake-version-specific, so pin
+      - name: Get CMake 4.5+ and ninja
-      # 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
         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.

.github/workflows/windows.yml

@@ -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
+      # The module test uses `import std;`, which needs CMake 4.5 or newer.
-      # import-std support. Its opt-in token is CMake-version-specific, so pin
+      - name: Get CMake 4.5+ and ninja
-      # 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
         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/module_cpp20/CMakeLists.txt

@@ -1,11 +1,6 @@
-cmake_minimum_required(VERSION 3.30)
+# `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.
-# `import std;` (used by main.cpp) needs CMake's experimental C++ import-std
+cmake_minimum_required(VERSION 4.5)
-# 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)
 
@@ -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)
+// across GCC, Clang, and MSVC. This requires C++23 and CMake 4.5 or newer;
-// import-std support; see CMakeLists.txt.
+// see CMakeLists.txt.
 int main()
 {
     // basic_json / json: parsing and value access