x3::forward_ast ambiguous call

[mikhail.strelnikov@…]

Following code works for boost::variant but fails for boost::spirit::x3::variant with error: call of '(const c) (boost::spirit::x3::forward_ast<s1>&)' is ambiguous

#define USE_SPIRIT_X3_VARIANT 1

#include <boost/fusion/include/define_struct_inline.hpp>
#include <boost/spirit/home/x3/support/ast/variant.hpp>
#include <boost/variant.hpp>

struct s1;

#if USE_SPIRIT_X3_VARIANT == 1
using s2 = boost::spirit::x3::variant<long, boost::spirit::x3::forward_ast<s1> >;
#else
using s2 = boost::variant<long, boost::recursive_wrapper<s1> >;
#endif

BOOST_FUSION_DEFINE_STRUCT_INLINE
(
    s1,
    (s2, v1)
)

BOOST_FUSION_DEFINE_STRUCT_INLINE
(
    magic,
    (int*, v1)
)

struct c
{
    int operator()(long) const { return 0; }
    int operator()(s1) const { return 1; }
    int operator()(magic) const { return 3; }
};

int main()
{
    s2 s; s = s1{};
    return boost::apply_visitor(c(), s);
}

​http://melpon.org/wandbox/permlink/BPdeuyNPLINriCr3

As you can see, magic class does not belong to the variant, but still causes compilation error.

[Joel de Guzman]

Confirmed. I think it is a bug with BOOST_FUSION_DEFINE_STRUCT_INLINE. Use BOOST_FUSION_ADAPT_STRUCT as a workaround. I suggest filing a bug against fusion. Damien Buhl is maintaining the adapt/define macros. I also suggest making a minimal test case with no Spirit X3 dependencies. The main issue is about calling a function object with two (or more?) operator() which are both fusion defined structs. Please close this ticket after filing the new ticket.

[mikhail.strelnikov@…]

But this example does not use Spirit X3 when USE_SPIRIT_X3_VARIANT macro is set to 0. If x3::variant is not used, this compiles and works as expected.

[Joel de Guzman]

And it also compiles if you use BOOST_FUSION_ADAPT_STRUCT which leads me to think that USE_SPIRIT_X3_VARIANT is a red herring. I'll see if I can make a minimal test.

[Kohei Takahashi <flast@…>]

Both of ADAPT_STRUCT and _INLINE have sequence conversion ctor, thus ADAPT_STRUCT doesn't help in this case as same error ​http://melpon.org/wandbox/permlink/zVVrJ2TWHhRy76re .

[Joel de Guzman]

I'm confused. That one uses BOOST_FUSION_DEFINE_STRUCT. This one compiles: ​http://melpon.org/wandbox/permlink/DSACLZOSLtg27kis

[Kohei Takahashi <flast@…>]

Oops, it's my bad, sorry. You are right.