[llvm-bugs] [Bug 38904] New: thread-safety-analysis doesn't know about std::optional

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38904

            Bug ID: 38904
           Summary: thread-safety-analysis doesn't know about
                    std::optional
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: lukasza at chromium.org
                CC: llvm-bugs at lists.llvm.org

Repro source code: https://godbolt.org/z/2XG89f (most important parts are
inlined below):

    void test2() {
      std::optional<AutoLock> maybe_autolock;
      maybe_autolock.emplace(AutoLock(g_lock));
      g_value += 1;
    }

For real code where this bug would have mattered see
https://crbug.com/881875#c1 and
services/ws/public/cpp/gpu/context_provider_command_buffer.cc in Chromium which
calls base::Optional<base::AutoLock>::emplace(base::Lock&).

Unexpected error:
    <source>:32:3: error: writing variable 'g_value' requires holding mutex
'g_lock' exclusively [-Werror,-Wthread-safety-analysis]

      g_value += 1;
      ^

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