id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
13479	GCC-warning: break strict-aliasing rules in <boost/type_traits/integral_constant.hpp>	ibiatiroler@…	John Maddock	"Currently, I receive the GCC-warning 'warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]' about the (indirect included) header-file <boost/boost/type_traits/integral_constant.hpp> (e.g. line 93), if:
 * I compile with GCC 4.6.4 or 6.4.0, and
 * boost 1.60.0 or 1.65.1, and
 * the option -Wstrict-aliasing=2 is used; not with on level 1 or 3
 * and optimization-level -O2 or -Os (or above), not with -O0,-O1 (or -Og on GCC 6)

IMHO, the reinterpret_cast from &data (I.e.g casting from const char*) causes this issue (and not using the dereference-helper afterwards, which comment shows that this is a GCC-work-around).

As work-around I'm using an additional const void* - helper-variable (ptr) instead of using &data directly, which seems to satisfy the compiler:

{{{
  operator const mpl::bool_<val>& ()const
  {
    static const char data = 0;
    void const* ptr = &data;
    return dereference(reinterpret_cast<const mpl::bool_<val>*>(ptr));
  }
}}}

... similar code-snippet also for 'operator const mpl::integral_c<T, val>& ()const' on line 65 ...
[Note: using 'void const* const ptr =&data;' does not fix it]

As alternative, an additional cast '(uintptr_t)&data' within reinterpret_cast seems to fix/inhibit this compiler-warning also.

At least, is my fix/work-around a feasible solution (without any side-effects)?

Best regards from Salzburg,
Markus

"	Bugs	closed	To Be Determined	type_traits	Boost 1.65.0	Problem	fixed	GCC