id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
3021,proto::extends compilation error with function type on gcc-3.4.5,François Barel <frabar666@…>,Eric Niebler,"The following test case has been simplified from Spirit's actions.cpp test case. This does not compile with gcc-3.4.5 or 3.4.6:

{{{
#include <boost/proto/proto.hpp>
using namespace boost::proto;

void func() {}

struct my_tag {};
struct my_terminal :
    extends< terminal<my_tag>::type, my_terminal >
{};
my_terminal const foo = my_terminal();

int main()
{
    foo[&func]; // OK
    foo[func];  // fails with gcc-3.4.5 and 3.4.6
    return 0;
}
}}}

with this error:

{{{
boost/proto/extends.hpp: In member function '...':
test.cpp:15:   instantiated from here
boost/proto/extends.hpp:505: error: call of overloaded `as_child(void (&)())' is ambiguous
boost/proto/traits.hpp:2050: note: candidates are: typename result_of::as_child<T, Domain, void>::type as_child(T&, typename disable_if<is_const<T>, detail::undefined>::type*) [with Domain = default_domain, T = void ()()]
boost/proto/traits.hpp:2059: note:                 typename result_of::as_child<const T, Domain, void>::type as_child(const T&) [with Domain = default_domain, T = void ()()]
}}}

(but it works fine with gcc-4.1.2 and later).

[[BR]]

I think it is because these versions of gcc do not implement DR-214 resolution ([http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#214 Partial ordering of function templates], see [http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19203 gcc bug #19203]), so:
 * since the disable_if on as_child does not disable the non-const overload (since the function type is not const),
 * gcc reports the 2 overloads as ambiguous instead of choosing the const one.

A solution would probably be to also disable the non-const as_child overload if the argument type is a function?

Thanks,[[BR]]
François",Bugs,closed,Boost 1.40.0,proto,Boost Development Trunk,Problem,fixed,proto extends function gcc DR214,