The following code fails to compile with GCC 4.9.2 on Linux and OS X: g++-4.9 -std=c++11 test.cc I was not able to reduce the code any further: * Commenting out 4, 5, 6, 7 or 11 causes the code to compile. * Changing 'C c = C()' to 'C c' or 'C c; c = C()' also causes the code to compile. Code: #include <functional> struct C { C() = default; C(std::function<C(int)>); C(std::function<C(int, int)>); template <class T> C operator () (T&&); template <class T> C operator () (T&&, T&&); }; int main() { C c = C(); } Error output: In file included from /opt/gcc-4.9.2/include/c++/4.9.2/bits/move.h:57:0, from /opt/gcc-4.9.2/include/c++/4.9.2/bits/stl_pair.h:59, from /opt/gcc-4.9.2/include/c++/4.9.2/utility:70, from /opt/gcc-4.9.2/include/c++/4.9.2/tuple:38, from /opt/gcc-4.9.2/include/c++/4.9.2/functional:55, from test-gcc.cc:1: /opt/gcc-4.9.2/include/c++/4.9.2/type_traits: In instantiation of ‘struct std::__and_<std::__not_<std::is_same<C, std::function<C(int, int)> > >, std::__or_<std::is_void<C>, std::is_convertible<C, C> > >’: /opt/gcc-4.9.2/include/c++/4.9.2/functional:2163:63: required by substitution of ‘template<class _Res, class ... _ArgTypes> template<class _Cond, class _Tp> using _Requires = typename std::enable_if<_Cond:: value, _Tp>::type [with _Cond = std::__and_<std::__not_<std::is_same<C, std::function<C(int, int)> > >, std::__or_<std::is_void<C>, std::is_convertible<C, C> > >; _Tp = void; _Res = C; _ArgTypes = {int, int}]’ /opt/gcc-4.9.2/include/c++/4.9.2/functional:2225:9: required by substitution of ‘template<class _Functor, class> std::function<_Res(_ArgTypes ...)>::function(_Functor) [with _Functor = C; <template-parameter-1-2> = <missing>]’ /opt/gcc-4.9.2/include/c++/4.9.2/type_traits:1401:39: required by substitution of ‘template<class _From1, class _To1, class> static std::true_type std::__is_convertible_helper<_From, _To, false>::__test(int) [with _From1 = C; _To1 = C; <template-parameter-1-3> = <missing>]’ /opt/gcc-4.9.2/include/c++/4.9.2/type_traits:1410:30: required from ‘struct std::__is_convertible_helper<C, C, false>’ /opt/gcc-4.9.2/include/c++/4.9.2/type_traits:1416:12: required from ‘struct std::is_convertible<C, C>’ /opt/gcc-4.9.2/include/c++/4.9.2/type_traits:111:12: required from ‘struct std::__or_<std::is_void<C>, std::is_convertible<C, C> >’ /opt/gcc-4.9.2/include/c++/4.9.2/type_traits:134:12: required from ‘struct std::__and_<std::__not_<std::is_same<C, std::function<C(int)> > >, std::__or_<std::is_void<C>, std::is_convertible<C, C> > >’ /opt/gcc-4.9.2/include/c++/4.9.2/functional:2163:63: required by substitution of ‘template<class _Res, class ... _ArgTypes> template<class _Cond, class _Tp> using _Requires = typename std::enable_if<_Cond:: value, _Tp>::type [with _Cond = std::__and_<std::__not_<std::is_same<C, std::function<C(int)> > >, std::__or_<std::is_void<C>, std::is_convertible<C, C> > >; _Tp = void; _Res = C; _ArgTypes = {int}]’ /opt/gcc-4.9.2/include/c++/4.9.2/functional:2225:9: required by substitution of ‘template<class _Functor, class> std::function<_Res(_ArgTypes ...)>::function(_Functor) [with _Functor = C; <template-parameter-1-2> = <missing>]’ test-gcc.cc:14:13: required from here /opt/gcc-4.9.2/include/c++/4.9.2/type_traits:134:12: error: invalid use of incomplete type ‘std::conditional<true, std::__or_<std::is_void<C>, std::is_convertible<C, C> >, std::__not_<std::is_same<C, std::function<C(int, int)> > > >::type {aka struct std::__or_<std::is_void<C>, std::is_convertible<C, C> >}’ struct __and_<_B1, _B2> ^ /opt/gcc-4.9.2/include/c++/4.9.2/type_traits:111:12: error: declaration of ‘std::conditional<true, std::__or_<std::is_void<C>, std::is_convertible<C, C> >, std::__not_<std::is_same<C, std::function<C(int, int)> > > >::type {aka struct std::__or_<std::is_void<C>, std::is_convertible<C, C> >}’ struct __or_<_B1, _B2>
The problem is that 'C c = C();' has to check for an accessible copy constructor, which needs to perform overload resolution on the constructors of C. That means checking if the std::function<C(...)> arguments can be initialized from an rvalue of type C. std::function has SFINAE constraints to check if it is being constructed from a suitable type, and those constrraints require a complete type. This fixes it by short-circuiting the constraints, but this might have unwanted consequences: --- include/c++/5.0.0/functional.orig 2015-04-15 12:54:33.198940049 +0100 +++ include/c++/5.0.0/functional 2015-04-15 12:54:35.671952823 +0100 @@ -1962,7 +1962,7 @@ template<typename _From, typename _To> using __check_func_return_type - = __or_<is_void<_To>, is_convertible<_From, _To>>; + = __or_<is_void<_To>, is_same<_From, _To>, is_convertible<_From, _To>>; /** * @brief Primary class template for std::function.
Author: redi Date: Wed Apr 29 16:07:47 2015 New Revision: 222581 URL: https://gcc.gnu.org/viewcvs?rev=222581&root=gcc&view=rev Log: PR libstdc++/65760 * include/std/functional (__check_func_return_type): Use is_same to avoid using _is_convertible on incomplete types. * testsuite/20_util/function/65760.cc: New. Added: trunk/libstdc++-v3/testsuite/20_util/function/65760.cc Modified: trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/include/std/functional
Fixed on trunk.