boost::atomic< char* > generates warning on fetch_add on gcc 4.8+

[Matt Hauck <matthauck@…>]

I have a boost::atomic< char* > object, on which I am calling fetch_add with ptrdiff_t, and apparently only on gcc 4.8+, I am getting the following warning:

In file included from /home/foo/packages/boost_1_61_0/include/boost/atomic/detail/atomic_template.hpp:25:0,
                 from /home/foo/packages/boost_1_61_0/include/boost/atomic/atomic.hpp:22,
                 from /home/foo/packages/boost_1_61_0/include/boost/atomic.hpp:12,
                 from /home/foo/code/ThingThatUsesAtomic.h:0,
/home/foo/packages/boost_1_61_0/include/boost/atomic/detail/bitwise_cast.hpp: In instantiation of ‘To boost::atomics::detail::bitwise_cast(const From&) [with To = char*; From = long unsigned int]’:
/home/foo/packages/boost_1_61_0/include/boost/atomic/detail/atomic_template.hpp:449:59:   required from ‘T* boost::atomics::detail::base_atomic<T*, void*>::fetch_add(boost::atomics::detail::base_atomic<T*, void*>::difference_type, boost::memory_order) volatile [with T = char; boost::atomics::detail::base_atomic<T*, void*>::value_type = char*; boost::atomics::detail::base_atomic<T*, void*>::difference_type = long int]’
[ actual code references redacted ... ]
/home/foo/packages/boost_1_61_0/include/boost/atomic/detail/bitwise_cast.hpp:41:5: warning: casting ‘char*’ to ‘char&’ does not dereference pointer

[Andrey Semashev]

Should be fixed in ​https://github.com/boostorg/atomic/commit/e6e6a04114997cc96a3c71d18933d5e19f2875e0.