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 )