proto::extends compilation error with function type on gcc-3.4.5

[François Barel <frabar666@…>]

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).

I think it is because these versions of gcc do not implement DR-214 resolution (​Partial ordering of function templates, see ​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,
François

[Eric Niebler]

(In [53158]) workaround for gcc-3.4 bug, fixes #3021