id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
13619	boost.crc fails to compile with gcc due to undefined behaviour	Pavel Fevralev <dllmain@…>	Daryle Walker	"Boost revision: 62faaa4584e16cd60cd62d5cbb0833954686e89c\\
Following code is minimal example to reproduce error:
{{{
 #include <boost/crc.hpp>
 template class boost::crc_optimal<6,6,6,6,0,0>;
 int main() {}
}}}
{{{
 $ g++ -std=c++14 -pedantic-errors -I../boost/install-root/include/ -o/dev/null crc-test.cxx
 In file included from ../boost/install-root/include/boost/config.hpp:61:0,
                  from ../boost/install-root/include/boost/crc.hpp:12,
                  from crc-test.cxx:1:
 ../boost/install-root/include/boost/crc.hpp: In instantiation of 'const least boost::detail::mask_uint_t<6ul>::sig_bits':
 ../boost/install-root/include/boost/crc.hpp:356:9:   required from 'const fast boost::detail::mask_uint_t<6ul>::sig_bits_fast'
 ../boost/install-root/include/boost/crc.hpp:915:41:   required from 'boost::crc_optimal<Bits, TruncPoly, InitRem, FinalXor, ReflectIn, ReflectRem>::value_type boost::crc_optimal<Bits, TruncPoly, InitRem, FinalXor, ReflectIn, ReflectRem>::get_interim_remainder() const [with long unsigned int Bits = 6ul; typename boost::uint_t<Bits>::fast TruncPoly = 6u; typename boost::uint_t<Bits>::fast InitRem = 6u; typename boost::uint_t<Bits>::fast FinalXor = 6u; bool ReflectIn = false; bool ReflectRem = false; boost::crc_optimal<Bits, TruncPoly, InitRem, FinalXor, ReflectIn, ReflectRem>::value_type = unsigned char]'
 crc-test.cxx:3:23:   required from here
 ../boost/install-root/include/boost/crc.hpp:350:69: error: left operand of shift expression '(-1 << 6ul)' is negative [-fpermissive]
          BOOST_STATIC_CONSTANT( least, sig_bits = (~( ~(least( 0u )) << Bits )) );
                                                     ~~~~~~~~~~~~~~~~~^~~~~~~~~
}}}
GCC produce incorrect message: there is no -Werror options or similar, as you see. Reason is initialization of static constant in declaration in class with expression that is _not_ constant because consists UB(left shift of negative value). '0u' literal is unsigned, but promouted to 'int' when 'least' is 'unsigned char'. Error is reproductable on release 1.67.0. Solution, for example, is make explicit cast of left operand to unsigned integral type. GCC information:
{{{
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 6.3.0-18+deb9u1' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 6.3.0 20170516 (Debian 6.3.0-18+deb9u1)
}}}"	Bugs	new	To Be Determined	crc	Boost 1.67.0	Problem