[llvm-bugs] [Bug 52413] New: [C++20] [Module] Emit an error if user enables both c++20 module and clang module

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52413

            Bug ID: 52413
           Summary: [C++20] [Module] Emit an error if user enables both
                    c++20 module and clang module
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: yedeng.yd at linux.alibaba.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

See https://lists.llvm.org/pipermail/cfe-dev/2021-November/069237.html for
details.
Simply, since c++20 module and clang module share the base infrastructure but
they have different semantics. It should be clear what's the behavior if the
user enables both c++20 module and clang module.

According to the discussion, the wanted behavior may be:
- If the user specify both c++20 module and clang module, emit an error.
- If the user want to use other feature other c++20 module in c++20 with clang
module, the user could make it by `-std=c++20 -fno-cxx-modules -fmodules`.

(Renaming `-fmodules` to `-fclang-modules` is another proposal. But it has a
bigger impact. Try to handle it in other PR)

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