Support for /Zc:preprocessor in clang-cl

[evg656e]

There is a new option for MSVC that makes the preprocessor standard-compliant: Microsoft Documentation. As of LLVM 20.1.6, clang-cl ignores this option, which allows the writing of ill-formed macros. It be nice to have this option working in clang-cl.

Other reports can be found here: Stack Overflow.

[AaronBallman]

clang-cl ignores the option because we only support a conforming preprocessor mode. The linked Stack Overflow is a bit confusing -- clang-cl produces the same output as cl with /Zc:preprocessor: https://godbolt.org/z/jfvr656be

static inline int func(int x) { return x; }

int main(void)
{
    return ({func(3);});
}

Is this a request to support /Zc:preprocessor- in clang-cl so we match MSVC's behavior?

[evg656e]

Here is another example where behavior diverges from MSVC with /Zc:preprocessor and even from default clang driver: https://godbolt.org/z/43hWfbbPT

[AaronBallman]

Thanks for the example! That is a difference between the two compilers.

Clang, in -fms-compatibility mode (which is enabled by default in clang-cl), produces:

template <typename... Args>
void print(const char* fmt, Args... args)
{
    print(fmt, args...);
}

int main(void)
{
    if (true) { print("print with arg %d", 42); };
    if (true) { print("print no args"); };
}

which is the same output you get from cl with /Zc:preprocessor-.

Clang, when not in -fms-compatibility mode, produces:

template <typename... Args>
void print(const char* fmt, Args... args)
{
    print(fmt, args...);
}

int main(void)
{
    if (true) { print("print with arg %d", 42); };
    if (true) { print("print no args", ); };
}

which is the same output you get from cl with /Zc:preprocessor (note the trailing comma in the second call to print).

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eugene (evg656e)

There is a new option for MSVC that makes the preprocessor standard-compliant: [Microsoft Documentation](https://learn.microsoft.com/en-us/cpp/build/reference/zc-preprocessor?view=msvc-170). As of LLVM 20.1.6, clang-cl ignores this option, which allows the writing of ill-formed macros. It be nice to have this option working in clang-cl.

Other reports can be found here: Stack Overflow.