Compile features ensure a compiler is aware of a standard version, but the one
actually used may be newer. The test relies on the standard chosen being
pre-C++20, so force C++17 explicitly.
This was exposed by a nightly bot that has a compiler defaulting to C++23. The
test would've broken anyway in a few years once GCC or Clang upped their
default.
This includes a number of examples that should work for various levels
of support in a compiler.
There are a number of tests which are gated on various features in the
compilers. To enable the tests, set `CMake_TEST_MODULE_COMPILATION` to a
comma-separated (to avoid `;`-escaping problems) to the list of features
which are supported:
- `named`: Named modules are supported.
- `shared`: Shared libraries with module usage at the API boundary are
supported.
- `partitions`: Named module partitions are supported.
- `internal_partitions`: Named module internal partitions are
supported.
Additionally, a `CMake_TEST_MODULE_COMPILATION_RULES` file must be
passed which contains the rules for how to build modules using the
provided compiler. It will be included in the tests to provide these
rules. To verify that the file provided works as intended, it must set
`CMake_TEST_CXXModules_UUID` to a specific version to indicate that it
is an expected file.
C++ modules have two variants which are of importance to CMake:
- `CXX_MODULES`: interface modules (those using `export module M;`,
`export module M:part;`, or `module M:internal_part;`)
- `CXX_MODULE_HEADER_UNITS`: importable header units
Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).
