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>

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