Library not usable under ThreadSanitizer with clang < 5, gcc == 6

[anarthal]

The problem is that the sanitizer interface was added in a later version than the sanitizer flags themselves. Reproducer:

#if !defined(BOOST_ASIO_HAS_THREAD_SANITIZER)
#if defined(__SANITIZE_THREAD__)
#define BOOST_ASIO_HAS_THREAD_SANITIZER 1
#elif defined(__has_feature)
#if __has_feature(thread_sanitizer)
#define BOOST_ASIO_HAS_THREAD_SANITIZER 1
#endif // __has_feature(thread_sanitizer)
#endif // defined(__SANITIZE_THREAD__)
#endif // !defined(BOOST_ASIO_HAS_THREAD_SANITIZER)

#include <iostream>

#if defined(BOOST_ASIO_HAS_THREAD_SANITIZER)
#include <sanitizer/tsan_interface.h>
#endif // defined(BOOST_ASIO_HAS_THREAD_SANITIZER)

int main() {
#ifdef BOOST_ASIO_HAS_THREAD_SANITIZER
  std::cout << "Has tsan: true\n";
#else
  std::cout << "Has tsan: false\n";
#endif
}

Build under clang 4.0 (Docker container ubuntu:16.04) and gcc 7 (Docker container ubuntu:18.04). The problem is that BOOST_ASIO_HAS_THREAD_SANITIZER is true, but the header sanitizer/tsan_interface.h does not exist.

Note that gcc 6 and lower has -fsanitize=thread but is not detected by any of the checks above. Builds using it report (hopefully) false positives.