Author: Chuanqi Xu Date: 2024-08-13T16:34:32+08:00 New Revision: 85b113c381d14d69bb6f1386488308859f74feaf
URL: https://github.com/llvm/llvm-project/commit/85b113c381d14d69bb6f1386488308859f74feaf DIFF: https://github.com/llvm/llvm-project/commit/85b113c381d14d69bb6f1386488308859f74feaf.diff LOG: [Doc] [C++20] [Modules] Clarify the reachability of internal partition units (#102572) Motivated by https://github.com/llvm/llvm-project/issues/101348 Although I don't want the tool's doc to explain the standard's wording, the wording itself has some unspecified thing. So I feel it will be helpful to make it clear. At least it may help us receive less invalid issue reports. Added: Modified: clang/docs/StandardCPlusPlusModules.rst Removed: ################################################################################ diff --git a/clang/docs/StandardCPlusPlusModules.rst b/clang/docs/StandardCPlusPlusModules.rst index 2478a77e7640c5..ccc0cb59f8e710 100644 --- a/clang/docs/StandardCPlusPlusModules.rst +++ b/clang/docs/StandardCPlusPlusModules.rst @@ -1230,6 +1230,58 @@ parsing their headers, those should be included after the import. If the imported modules don't provide such a header, one can be made manually for improved compile time performance. +Reachability of internal partition units +---------------------------------------- + +The internal partition units are sometimes called implementation partition units in other documentation. +However, the name may be confusing since implementation partition units are not implementation +units. + +According to `[module.reach]p1 <https://eel.is/c++draft/module.reach#1>`_ and +`[module.reach]p2 <https://eel.is/c++draft/module.reach#2>`_ (from N4986): + + A translation unit U is necessarily reachable from a point P if U is a module + interface unit on which the translation unit containing P has an interface + dependency, or the translation unit containing P imports U, in either case + prior to P. + + All translation units that are necessarily reachable are reachable. Additional + translation units on which the point within the program has an interface + dependency may be considered reachable, but it is unspecified which are and + under what circumstances. + +For example, + +.. code-block:: c++ + + // a.cpp + import B; + int main() + { + g<void>(); + } + + // b.cppm + export module B; + import :C; + export template <typename T> inline void g() noexcept + { + return f<T>(); + } + + // c.cppm + module B:C; + template<typename> inline void f() noexcept {} + +The internal partition unit ``c.cppm`` is not necessarily reachable by +``a.cpp`` because ``c.cppm`` is not a module interface unit and ``a.cpp`` +doesn't import ``c.cppm``. This leaves it up to the compiler to decide if +``c.cppm`` is reachable by ``a.cpp`` or not. Clang's behavior is that +indirectly imported internal partition units are not reachable. + +The suggested approach for using an internal partition unit in Clang is +to only import them in the implementation unit. + Known Issues ------------ _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
