#include <mutex>

std::mutex m0, m1;

void func() {
  std::unique_lock ul1(m0, std::defer_lock);
  std::unique_lock ul2(m1, std::defer_lock);
  std::lock(ul1, ul2);
}

$ ~/misc/clang-cipd-latest/bin/clang++ -c -Wall -Wthread-safety /tmp/test.cc
/tmp/test.cc:9:1: warning: mutex 'ul1' is still held at the end of function [-Wthread-safety-analysis]
    9 | }
      | ^
/tmp/test.cc:8:3: note: mutex acquired here
    8 |   std::lock(ul1, ul2);
      |   ^
/tmp/test.cc:9:1: warning: mutex 'ul2' is still held at the end of function [-Wthread-safety-analysis]
    9 | }
      | ^
/tmp/test.cc:8:3: note: mutex acquired here
    8 |   std::lock(ul1, ul2);
      |   ^
2 warnings generated.