Author: Chuanqi Xu Date: 2023-03-17T15:10:37+08:00 New Revision: 684955a2302daacb4136b0247c00f6f95472614b
URL: https://github.com/llvm/llvm-project/commit/684955a2302daacb4136b0247c00f6f95472614b DIFF: https://github.com/llvm/llvm-project/commit/684955a2302daacb4136b0247c00f6f95472614b.diff LOG: [Modules] [doc] Document the problem that we can't include headers before import declarations Added: Modified: clang/docs/StandardCPlusPlusModules.rst Removed: ################################################################################ diff --git a/clang/docs/StandardCPlusPlusModules.rst b/clang/docs/StandardCPlusPlusModules.rst index 970803b56c8c6..ab34ba03ba14c 100644 --- a/clang/docs/StandardCPlusPlusModules.rst +++ b/clang/docs/StandardCPlusPlusModules.rst @@ -616,6 +616,50 @@ and add the label ``clang:modules`` (if you have permissions for that). For higher level support for proposals, you could visit https://clang.llvm.org/cxx_status.html. +Including headers after import is problematic +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +For example, the following example can be accept: + +.. code-block:: c++ + + #include <iostream> + import foo; // assume module 'foo' contain the declarations from `<iostream>` + + int main(int argc, char *argv[]) + { + std::cout << "Test\n"; + return 0; + } + +but it will get rejected if we reverse the order of ``#include <iostream>`` and +``import foo;``: + +.. code-block:: c++ + + import foo; // assume module 'foo' contain the declarations from `<iostream>` + #include <iostream> + + int main(int argc, char *argv[]) + { + std::cout << "Test\n"; + return 0; + } + +Both of the above examples should be accepted. + +This is a limitation in the implementation. In the first example, +the compiler will see and parse <iostream> first then the compiler will see the import. +So the ODR Checking and declarations merging will happen in the deserializer. +In the second example, the compiler will see the import first and the include second. +As a result, the ODR Checking and declarations merging will happen in the semantic analyzer. + +So there is divergence in the implementation path. It might be understandable that why +the orders matter here in the case. +(Note that "understandable" is diff erent from "makes sense"). + +This is tracked in: https://github.com/llvm/llvm-project/issues/61465 + Ambiguous deduction guide ~~~~~~~~~~~~~~~~~~~~~~~~~ _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
