Where is the data race in this simple c++ code

Both clang++ and g++ sanitizers produce similar warning about data race for this simple code. Is it a false alarm? What is the problem?

Code:

#include <thread>
struct A
{
    void operator()()
    {
    }
};

struct B
{
    void operator()()
    {
    }
};

int main(void)
{
    // callable objects are created and moved into thread
    std::thread t1(A{});
    std::thread t2(B{});
    t1.join();
    t2.join();
    return 0;
}

Compile flags:

-pthread -O0 -g -fsanitize=thread -fsanitize=undefined

Sanitizer output for g++:

==================
WARNING: ThreadSanitizer: data race (pid=80173)
  Write of size 8 at 0x7b0400000800 by thread T2:
    #0 pipe ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1726 (libtsan.so.0+0x3ea28)
    #1 __sanitizer::IsAccessibleMemoryRange(unsigned long, unsigned long) ../../../../src/libsanitizer/sanitizer_common/sanitizer_posix_libcdep.cpp:276 (libubsan.so.1+0x20102)
    #2 std::thread::_State_impl<std::thread::_Invoker<std::tuple<B> > >::~_State_impl() /usr/include/c++/11/bits/std_thread.h:201 (a.out+0x5191)
    #3 <null> <null> (libstdc++.so.6+0xdc2cb)

  Previous write of size 8 at 0x7b0400000800 by thread T1:
    #0 pipe ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1726 (libtsan.so.0+0x3ea28)
    #1 __sanitizer::IsAccessibleMemoryRange(unsigned long, unsigned long) ../../../../src/libsanitizer/sanitizer_common/sanitizer_posix_libcdep.cpp:276 (libubsan.so.1+0x20102)
    #2 std::thread::_State_impl<std::thread::_Invoker<std::tuple<A> > >::~_State_impl() /usr/include/c++/11/bits/std_thread.h:201 (a.out+0x53a5)
    #3 <null> <null> (libstdc++.so.6+0xdc2cb)

  Thread T2 (tid=80176, running) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:969 (libtsan.so.0+0x605b8)
    #1 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) <null> (libstdc++.so.6+0xdc398)
    #2 main a.cpp:20 (a.out+0x3396)

  Thread T1 (tid=80175, finished) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:969 (libtsan.so.0+0x605b8)
    #1 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) <null> (libstdc++.so.6+0xdc398)
    #2 main a.cpp:19 (a.out+0x3383)

SUMMARY: ThreadSanitizer: data race ../../../../src/libsanitizer/sanitizer_common/sanitizer_posix_libcdep.cpp:276 in __sanitizer::IsAccessibleMemoryRange(unsigned long, unsigned long)
==================
ThreadSanitizer: reported 1 warnings

Note: This warning is given only when thread and UB sanitizers are both enabled.

The program is well-formed. It doesn't have any data race or other undefined behavior and it also doesn't have any race condition or unspecified behavior (except for the possibility of aborting with an uncaught exception if thread creation fails).

Thread sanitizer is simply not playing nice with the undefined behavior sanitizer. Whether they are meant to be usable together I am not sure. I have had issues like this before when combining them and so would recommend not doing that.

If they are meant to play nicely together, then this would indeed be a bug.