clang-cl behaves different on attributes than MSVC

[Jimbopython]

When using the attribute [[nodiscard]] after __declspec(dllexport), the compiler complains:

<source>(5,25): error: 'nodiscard' attribute cannot be applied to types

When swapping the [[nodiscard]] and __declspec(dllexport), the compilation succeeds.

This behavior is different from MSVC, as MSVC does not care about the order of the arguments.

Tested with

• clang-cl 20.1.8 on Ubuntu
• clang-cl 18.1.0 Godbolt
• MSVC v19.20 Godbolt

Complete example:

enum bar {
    bar1 = 0
};

__declspec(dllexport) [[nodiscard]] bar foo() {
    return bar::bar1;
}


int main() {
    foo();
    return 0;
}

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Jimbopython)

When using the attribute `[[nodiscard]]` after `__declspec(dllexport)`, the compiler complains: ``` <source>(5,25): error: 'nodiscard' attribute cannot be applied to types ``` When swapping the `[[nodiscard]]` and `__declspec(dllexport)`, the compilation succeeds.

This behavior is different from MSVC, as MSVC does not care about the order of the arguments.

Tested with

• clang-cl 20.1.8 on Ubuntu
• clang-cl 18.1.0 Godbolt
• MSVC v19.20 Godbolt

Complete example:

enum bar {
    bar1 = 0
};

__declspec(dllexport) [[nodiscard]] bar foo() {
    return bar::bar1;
}


int main() {
    foo();
    return 0;
}

[AaronBallman]

I didn't have the chance to fully debug the issue, but a quick look suggests the issue is confusion as to what the attributes should be attached to, the DeclSpec, the Declarator, or something else. We put both attributes into the DeclSpec and then attempt to process them as type attributes, hitting this code: https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaType.cpp#L8879 because dllexport is not a standard attribute or keyword, we don't diagnose, but [[nodiscard]] is a standard attribute and so it is diagnosed. I don't think either should be processed as a type attribute in that position though, they should apply to the whole declaration group.