Bug#805820: -Wuninitialized provokes unexpected error with LTO

[Richard Kettlewell <rjk@greenend.org.uk>]

Package: g++-5
Version: 5.2.1-23

$ type g++-5
g++-5 is /usr/bin/g++-5
$ dpkg -l g++-5 libstdc++-5-dev
Desired=Unknown/Install/Remove/Purge/Hold
|
Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version      Architecture Description
+++-==============-============-============-=================================
ii  g++-5          5.2.1-23     i386         GNU C++ compiler
ii  libstdc++-5-de 5.2.1-23     i386         GNU Standard C++ Library v3
(deve
$ uname -a
Linux deodand 4.2.0-1-686-pae #1 SMP Debian 4.2.3-2 (2015-10-14) i686
GNU/Linux
$ cat t.cc
#include <regex>
#include <string>

int main() {
  std::string a;
  std::regex r("x");
  return std::regex_match(a, r);
}
$ g++-5 -O2 -std=c++11 -Wuninitialized -c t.cc
$ g++-5 -O2 -std=c++11 -Wuninitialized -o t t.o
$ g++-5 -O2 -flto -fuse-linker-plugin -std=c++11 -Wuninitialized -c t.cc
$ g++-5 -O2 -flto -fuse-linker-plugin -std=c++11 -Wuninitialized -o t t.o
/usr/include/c++/5/bits/regex_compiler.tcc: In member function
‘_M_alternative’:
/usr/include/c++/5/bits/regex_compiler.tcc:266:8: warning: ‘__n’ may be
used uninitialized in this function [-Wmaybe-uninitialized]
        for (long __i = 0; __i < __n; ++__i)
        ^
/usr/include/c++/5/bits/regex_compiler.tcc:230:9: note: ‘__n’ was
declared here
    long __n;
         ^

The relevant bit of regex_compiler.tcc looks OK to me so I think this is
the compiler rather than the std::regex implementation.  ICBW.

ttfn/rjk