[Boost-bugs] [Boost C++ Libraries] #3021: proto::extends compilation error with function type on gcc-3.4.5

Subject: [Boost-bugs] [Boost C++ Libraries] #3021: proto::extends compilation error with function type on gcc-3.4.5
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2009-05-13 06:08:08

#3021: proto::extends compilation error with function type on gcc-3.4.5
--------------------------------------------------+-------------------------
 Reporter:  François Barel <frabar666_at_[hidden]>  |       Owner:  eric_niebler
     Type:  Bugs                                  |      Status:  new         
Milestone:  Boost 1.40.0                          |   Component:  proto       
  Version:  Boost Development Trunk               |    Severity:  Problem     
 Keywords:  proto extends function gcc DR214      |  
--------------------------------------------------+-------------------------
 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

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/3021>
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