[Bug libstdc++/22207] New: Spurious 'might be used uninitialized' warnings in STL headers with -O2

["brian at dessent dot net" <gcc-bugzilla@gcc.gnu.org>]

The following reduced test case causes spurious -Wuninitialized warnings, but
only with -O2:

-----

#include <vector>
#include <string>

class OptionSet
{
public:
  OptionSet ();
  std::vector<std::string> const &nonOptions() const;
private:
  std::vector<std::string> nonoptions;
};

OptionSet::OptionSet()
{
  nonoptions = std::vector<std::string> ();
}

std::vector<std::string> const &
OptionSet::nonOptions() const
{
  return nonoptions;
}

-----

$ g++ -Wuninitialized -O2 -c spurious_uninitialized_testcase.cc
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_vector.h: In member
function `std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const
std::vector<_Tp, _Alloc>&) [with _Tp = std::string, _Alloc =
std::allocator<std::string>]':
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_vector.h:715: warning:
'__result' might be used uninitialized in this function
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_uninitialized.h:82:
warning: '__cur' might be used uninitialized in this function
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_uninitialized.h:82:
warning: '__cur' might be used uninitialized in this function

$ g++ -v
Reading specs from /usr/lib/gcc/i686-pc-cygwin/3.4.4/specs
Configured with: /gcc/gcc-3.4.4/gcc-3.4.4-1/configure --verbose --prefix=/usr
--exec-prefix=/usr --sysconfdir=/etc --libdir=/usr/lib --libexecdir=/usr/lib
--mandir=/usr/share/man --infodir=/usr/share/info
--enable-languages=c,ada,c++,d,f77,java,objc --enable-nls
--without-included-gettext --enable-version-specific-runtime-libs --without-x
--enable-libgcj --disable-java-awt --with-system-zlib --enable-interpreter
--disable-libgcj-debug --enable-threads=posix --enable-java-gc=boehm
--disable-win32-registry --enable-sjlj-exceptions --enable-hash-synchronization
--enable-libstdcxx-debug : (reconfigured) 
Thread model: posix
gcc version 3.4.4 (cygming special) (gdc 0.12, using dmd 0.125)

-----

The warnings only occur at -O2 and seem to have something to do with inlining. 
This causes our build that uses "-Wall -Werror" to die whereas it had no problem
with this warning in gcc-3.3.

I'm sensitive to the fact that it's impossible for the compiler to get this
warning correct every time, and the reason for the use of "might be" in the
warning text.  It's acceptible to emit warnings about things that may not
actually be the case, and normally I would just modify the build environment to
disable that warning or lose -Werror.  But in this case, the code in question is
in the STL and is completely out of the hands of the user, and I don't think
it's unreasonable to expect to be able to compile STL-based code with -Werror -Wall.

-- 
           Summary: Spurious 'might be used uninitialized' warnings in STL
                    headers with -O2
           Product: gcc
           Version: 3.4.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: brian at dessent dot net
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-pc-cygwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22207