Hi list,
I'm observing different behavior in boost::optional and boost::variant
compared to their std c++17 versions. Using Boost 1.64
The following example fails to compile with -DUSE_BOOST using either gcc
7.3 or clang 6.0
#include
#include
#include <optional>
#include <variant>
struct A {};
struct B {};
struct C {};
#ifdef USE_BOOST
using V = boost::variant;
using OV = boost::optional<V>;
#else
using V = std::variant;
using OV = std::optional<V>;
#endif
OV f(B b)
{
return b;
}
Is this difference in behavior expected for such a simple example?
Is it possible to improve Boost to make this example compile?
Thanks in advance!
PS: the Boost only version works in VS2015, not sure why...
PPS: Try it in godbolt! https://godbolt.org/g/n4Vbr9
gcc 7.3 says
<source>: In function 'OV f(B)':
<source>:20:12: error: could not convert 'b' from 'B' to 'OV {aka
boost::optional >}'
clang 6.0.0 says
<source>:20:12: error: no viable conversion from returned value of type 'B'
to function return type 'OV' (aka 'optional >')
return b;
^
/opt/compiler-explorer/libs/boost_1_64_0/boost/optional/optional.hpp:796:5:
note: candidate constructor not viable: no known conversion from 'B' to
'boost::none_t' for 1st argument
optional( none_t none_ ) BOOST_NOEXCEPT : base(none_) {}
^
/opt/compiler-explorer/libs/boost_1_64_0/boost/optional/optional.hpp:800:5:
note: candidate constructor not viable: no known conversion from 'B' to
'boost::optional >::argument_type' (aka 'const
boost::variant &') for 1st argument
optional ( argument_type val ) : base(val) {}
^
/opt/compiler-explorer/libs/boost_1_64_0/boost/optional/optional.hpp:805:5:
note: candidate constructor not viable: no known conversion from 'B' to
'boost::optional >::rval_reference_type' (aka
'boost::variant &&') for 1st argument
optional ( rval_reference_type val ) : base( boost::forward<T>(val) )
^
/opt/compiler-explorer/libs/boost_1_64_0/boost/optional/optional.hpp:877:5:
note: candidate constructor not viable: no known conversion from 'B' to
'const boost::optional > &' for 1st argument
optional ( optional const& rhs ) : base( static_cast(rhs)
) {}
^
/opt/compiler-explorer/libs/boost_1_64_0/boost/optional/optional.hpp:882:2:
note: candidate constructor not viable: no known conversion from 'B' to
'boost::optional > &&' for 1st argument
optional ( optional && rhs )
