Avoid spurious warnings when gcc's -Wconversion is used

[Eelis]

Consider:

#include <optional>
#include <boost/safe_numerics/safe_integer.hpp>

using I = boost::safe_numerics::safe<int>;

struct X { I i; };

std::optional<I> f(X x)
{
    return x.i;
}

When asked to compile the above with -Wconversion, g++ gives:

t.cpp: In function ‘std::optional<boost::safe_numerics::safe_base<int, -2147483648, 2147483647, boost::safe_numerics::native, boost::safe_numerics::exception_policy<boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::ignore_exception> > > f(X)’:
t.cpp:10:14: warning: choosing ‘constexpr std::optional<_Tp>::optional(_Up&&) [with _Up = boost::safe_numerics::safe_base<int, -2147483648, 2147483647, boost::safe_numerics::native, boost::safe_numerics::exception_policy<boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::ignore_exception> >&; typename std::enable_if<std::__and_<std::__not_<std::is_same<std::optional<_Tp>, typename std::decay<_Up>::type> >, std::__not_<std::is_same<std::in_place_t, typename std::decay<_Up>::type> >, std::is_constructible<_Tp, _Up&&>, std::is_convertible<_Up&&, _Tp> >::value, bool>::type <anonymous> = 1; _Tp = boost::safe_numerics::safe_base<int, -2147483648, 2147483647, boost::safe_numerics::native, boost::safe_numerics::exception_policy<boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::ignore_exception> >]’ over ‘constexpr boost::safe_numerics::safe_base<Stored, Min, Max, P, E>::operator R() const [with R = std::optional<boost::safe_numerics::safe_base<int, -2147483648, 2147483647, boost::safe_numerics::native, boost::safe_numerics::exception_policy<boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::ignore_exception> > >; typename std::enable_if<(! boost::safe_numerics::is_safe<R>::value), int>::type <anonymous> = 0; Stored = int; Stored Min = -2147483648; Stored Max = 2147483647; P = boost::safe_numerics::native; E = boost::safe_numerics::exception_policy<boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::ignore_exception>]’ [-Wconversion]
    return x.i;
            ^
t.cpp:10:14: warning:   for conversion from ‘I’ {aka ‘boost::safe_numerics::safe_base<int, -2147483648, 2147483647, boost::safe_numerics::native, boost::safe_numerics::exception_policy<boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::ignore_exception> >’} to ‘std::optional<boost::safe_numerics::safe_base<int, -2147483648, 2147483647, boost::safe_numerics::native, boost::safe_numerics::exception_policy<boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::throw_exception, boost::safe_numerics::ignore_exception> > >’ [-Wconversion]
t.cpp:10:14: note:   because conversion sequence for the argument is better

[robertramey]

I'm not sure that this has anything to do with safe_numerics per se but rather using implicit conversion of member variable to wrapper type. I'll leave this here while I think about it some more. Here is the page describing the warning for this particular case: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html . I'm thinking that gcc is being overly picky here. See "For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator". Also checkout what other compilers do in this case.

For now I think the prudent thing to do is to replace the implicit conversion with a more explicit one.

#include <optional>
#include <boost/safe_numerics/safe_integer.hpp>

using I = boost::safe_numerics::safe<int>;

struct X { I i; };

std::optional<I> f(X x)
{
    return std::optional<I>(x.i);
}

[Eelis]

I'm not sure that this has anything to do with safe_numerics per se but rather using implicit conversion of member variable to wrapper type.

If I use using I = int; or struct I{}; instead, the warning goes away. That made me think the issue had something to do with safe_numerics, but if that's not the case, feel free to close. :)