[llvm-bugs] [Bug 44137] New: __has_c_attribute in C++ mode

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44137

            Bug ID: 44137
           Summary: __has_c_attribute in C++ mode
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: aaron at aaronballman.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Currently, __has_c_attribute is a defined macro in C++ mode and responds with
nonzero values for attributes known to C++.

e.g.,

// In C++ mode
#if defined(__has_c_attribute)
#error "ugh" // This error triggers
#endif

// In C mode
#if defined(__has_cpp_attribute)
#error "ugh" // This error does not trigger
#endif

As shown in: https://godbolt.org/z/KnyQ2G

Additionally, it seems like the behavior of C-only attribute testing is broken,
even in C mode:

#if __has_c_attribute(clang::transparent_union)
#error "ugh" // This does not trigger, even in C mode
#endif

As shown in: https://godbolt.org/z/8eHs8R

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