Description of problem:

Code that previously compiled without warning on f25 gcc 6.3.1 now generates the warning in the summary.


Version-Release number of selected component (if applicable):
gcc-c++-7.1.1-1.fc26.x86_64

How reproducible:
100%

Steps to Reproduce:
$ lsb_release -r
Release:        25
$ gcc --version
gcc (GCC) 6.3.1 20161221 (Red Hat 6.3.1-1)
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ cat << EOF > punning.cpp
#include <type_traits>
#include <mutex>

int main()
{
  std::aligned_storage<sizeof(std::mutex)>::type state_mutex_bytes;
  new (&state_mutex_bytes) std::mutex;
  std::mutex& r = *static_cast<std::mutex*>(static_cast<void*>(&state_mutex_bytes));
  (void)r;
}
EOF
$ g++ -Wall -O2 -c punning.cpp
$

$ lsb_release -r
Release:        26
$ gcc --version
gcc (GCC) 7.1.1 20170503 (Red Hat 7.1.1-1)
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ cat << EOF > punning.cpp
#include <type_traits>
#include <mutex>

int main()
{
  std::aligned_storage<sizeof(std::mutex)>::type state_mutex_bytes;
  new (&state_mutex_bytes) std::mutex;
  std::mutex& r = *static_cast<std::mutex*>(static_cast<void*>(&state_mutex_bytes));
  (void)r;
}
EOF
$ g++ -Wall -O2 -c punning.cpp
punning.cpp: In function ‘int main()’:
punning.cpp:8:83: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   std::mutex& r = *static_cast<std::mutex*>(static_cast<void*>(&state_mutex_bytes));
                                                                                   ^


Actual results:
punning.cpp: In function ‘int main()’:
punning.cpp:8:83: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   std::mutex& r = *static_cast<std::mutex*>(static_cast<void*>(&state_mutex_bytes));

Expected results:
No warning generated

Additional info:
I believe this is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80593

There is also a reproducer in the upstream bug bug I included mine because it is closer to the actual issue seen here, https://copr-be.cloud.fedoraproject.org/results/badone/Ceph-nightlies/fedora-26-x86_64/00546644-ceph/build.log.gz