Using clang's feature checking macros to detect existence of std::launder

Question

I have to compile the same code using different versions of clang. Since the code contains some c++17 features that are not supported by every version of clang, I want to check during compile time if they are supported. As far as I can see, clang's feature checking macros are the right way to go.

My problem specifically arises with std::launder.

I created this minimal example:

#include "iostream"

#if __has_builtin(__builtin_launder)
    void test() {
        int i = 42;
        std::cout << "Should compile: " << std::launder(&i) << std::endl;
    }
#else
    void test() {
        int i = 42;
        std::cout << "Should not even compile: " << std::launder(&i) << std::endl;
    }
#endif

int main(){
    test();
}

If I'm compiling it (clang version 6.0.0, libc++) using clang++ -std=c++1z -stdlib=libc++ -Wall -pedantic test3.cpp && ./a.out
The output is: Should not even compile: 0x7fff75116f64

Although std::launder is obviously supported, the builtin check does not work. Since it is the same check as in reviews llvm: Implement std::launder, I was assuming the check to be correct.

What am I missing? I feel like it is something super simple, but I'm not seeing it.

Worst case I'd use cmake's try-compile mechanism for this purpose instead, but it seems to be an overkill and I'm still interested in finding out what the actual problem is.

Answer 1

From the link to the review you gave, in the implementation of (the library function) std::launder:

#ifdef _LIBCPP_COMPILER_HAS_BUILTIN_LAUNDER 
  return __builtin_launder(__p); 
#else
  return __p;
#endif
}

std::launder is there also if there's no builtin. Thus the existence or non-existence of the builtin won't tell you whether std::launder is there or not.

---

To test whether you have std::launder or not (which seems to be what you want) you can either use your configuration system (cmake, autoconf, ...) or try your luck with the new (C++17) feature tests:

#include <new> // important!
#if __cpp_lib_launder >= 201606
  // have std::launder for sure
#else
  // not sure, could've std::launder either way
#endif

Answer 2

See https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations . You are supposed to test __cpp_lib_launder, but libc++ doesn't seem to implement that in the version I have...