[llvm-bugs] [Bug 43584] New: clang-cl preprocesses variadic macros differently than clang and msvc

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43584

            Bug ID: 43584
           Summary: clang-cl preprocesses variadic macros differently than
                    clang and msvc
           Product: clang
           Version: 9.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: berglerma at gmail.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

For all version of clang I've tested (7.0.0, 8.0.0 and 9.0.0),

    #define THIRD(ARG_1, ARG_2, ARG_3, ...) ARG_3
    #define DEMO(...) THIRD(__VA_ARGS__, 3, 4, 5)

    int main() {
      return DEMO(,);
    }

turns into 

   int main() {
     return 3;
   }

when preprocessed with `clang -E demo.cpp`, but turns into

   int main() { 
     return 4;
   }

when preprocessed with `clang-cl /E demo.cpp`, regardless of whether clang-cl
is run on Linux or on Windows.

My first assumption was that clang-cl is using some kind of MSVC compatibility
mode for the preprocessor, but as it turns out it isn't doing the same thing as
msvc either: DEMO(1, 2) expands to 4 with MSVC, but to 3 with clang-cl.

Note in particular that DEMO(,) and DEMO(1, 2) expand to different results with
clang-cl, while that does not happen with MSVC.

This bug currently breaks googletest's MOCK_METHOD macro with clang-cl (see
https://github.com/google/googletest/issues/2490)

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