[Bug c++/101355] New: compiling coroutines with ubsan emits bogus -Wmaybe-uninitialized warnings

["daklishch at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101355

            Bug ID: 101355
           Summary: compiling coroutines with ubsan emits bogus
                    -Wmaybe-uninitialized warnings
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: daklishch at gmail dot com
  Target Milestone: ---

Created attachment 51111
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51111&action=edit
preprocessed source

Compiling the following program with UBSan emits -Wmaybe-uninitialized on
`<anonymous>' variables.
The compiler and runtime behavior is somewhat inconsistent. Normally, there is
only one warning per function and the program is not affected. However, for
example, compiling provided program with -O2 emits two identical warnings and
makes the program crash with `member call on null pointer of type 'struct a''.


$ cat testcode.cc
#include <coroutine>

struct coro {
        struct promise_type {
                coro get_return_object() { return {}; }
                std::suspend_never initial_suspend() noexcept { return {}; }
                std::suspend_never final_suspend() noexcept { return {}; }
                void unhandled_exception() {}
                void return_void() {}
        };

        bool await_ready() { return true; }
        void await_resume() {}
        template <typename U>
        void await_suspend(U &) {}
};

struct b {
        ~b() {}
};

struct a {
        a(b) {}
        ~a() {}
};

coro f(b obj) {
        auto obj2 = a{obj};
        co_return;
}

int main() {
        f({});
}
$ ~/gcc-dev/bin/gccdev -O2 -std=c++20 -Wall -fsanitize=undefined testcode.cc
-lstdc++
testcode.cc: In function ‘void _Z1f1b.actor(f(b)::_Z1f1b.frame*)’:
testcode.cc:30:1: warning: ‘<anonymous>’ may be used uninitialized
[-Wmaybe-uninitialized]
   30 | }
      | ^
testcode.cc:30:1: note: ‘<anonymous>’ was declared here
   30 | }
      | ^
In function ‘void _Z1f1b.actor(f(b)::_Z1f1b.frame*)’,
    inlined from ‘coro f(b)’ at testcode.cc:27:6:
testcode.cc:30:1: warning: ‘<anonymous>’ is used uninitialized
[-Wuninitialized]
   30 | }
      | ^
testcode.cc: In function ‘coro f(b)’:
testcode.cc:30:1: note: ‘<anonymous>’ was declared here
   30 | }
      | ^
$ ./a.out
testcode.cc:30:1: runtime error: member call on null pointer of type 'struct a'