https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61947
Bug ID: 61947
Summary: Ambiguous calls when constructing std::tuple
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: roman.perepelitsa at gmail dot com
#include <tuple>
struct ConvertibleToAny {
template <class T> operator T() const { return T(); }
};
int main() {
std::tuple<ConvertibleToAny&&> t(ConvertibleToAny{});
}
Produces:
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:262:4:
error: call to constructor of '_Head_base<0UL, (anonymous
namespace)::ConvertibleToAny &&, __empty_not_final<ConvertibleToAny
&&>::value>' is ambiguous
_Base(std::forward<_UHead>(__head)) { }
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:405:4:
note: in instantiation of function template specialization 'std::_Tuple_impl<0,
(anonymous namespace)::ConvertibleToAny &&>::_Tuple_impl<(anonymous
namespace)::ConvertibleToAny, void>' requested here
: _Inherited(std::forward<_UElements>(__elements)...) { }
^
experimental/users/romanp/tuple_bug/tuple-bug.cc:13:34: note: in instantiation
of function template specialization 'std::tuple<(anonymous
namespace)::ConvertibleToAny &&>::tuple<(anonymous
namespace)::ConvertibleToAny, void>' requested here
std::tuple<ConvertibleToAny&&> t(ConvertibleToAny{});
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:128:12:
note: candidate is the implicit move constructor
struct _Head_base<_Idx, _Head, false>
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:128:12:
note: candidate constructor (the implicit copy constructor) has been implicitly
deleted
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:142:7:
note: candidate constructor
_Head_base(__uses_alloc0)
The call shouldn't be ambiguous. It should pick the following constructor:
[tuple.cnstr]
template <class... UTypes>
explicit tuple(UTypes&&... u);
Requires: sizeof...(Types) == sizeof...(UTypes). is_constructible<Ti , Ui
&&>::value is true for all i.
Effects: Initializes the elements in the tuple with the corresponding value
in std::forward<UTypes>(u).
is_constructible<ConvertibleToAny&&, ConvertibleToAny&&>::value is true, so the
precondition is satisfied.
This bug affects std::bind as it's built on top of std::tuple.
#include <functional>
struct ConvertibleToAny {
template <class T> operator T() const { return T(); }
};
void Sink(int) {}
int main() {
std::bind(Sink, std::placeholders::_1)(ConvertibleToAny{});
}
Error:
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:262:4:
error: call to constructor of '_Head_base<0UL, (anonymous
namespace)::ConvertibleToAny &&, __empty_not_final<ConvertibleToAny
&&>::value>' is ambiguous
_Base(std::forward<_UHead>(__head)) { }
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:405:4:
note: in instantiation of function template specialization 'std::_Tuple_impl<0,
(anonymous namespace)::ConvertibleToAny &&>::_Tuple_impl<(anonymous
namespace)::ConvertibleToAny, void>' requested here
: _Inherited(std::forward<_UElements>(__elements)...) { }
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:868:14:
note: in instantiation of function template specialization
'std::tuple<(anonymous namespace)::ConvertibleToAny &&>::tuple<(anonymous
namespace)::ConvertibleToAny, void>' requested here
{ return tuple<_Elements&&...>(std::forward<_Elements>(__args)...); }
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/functional:1354:13:
note: in instantiation of function template specialization
'std::forward_as_tuple<(anonymous namespace)::ConvertibleToAny>' requested here
std::forward_as_tuple(std::forward<_Args>(__args)...),
^
experimental/users/romanp/tuple_bug/tuple-bug.cc:15:41: note: in instantiation
of function template specialization 'std::_Bind<void
(*(std::_Placeholder<1>))(int)>::operator()<(anonymous
namespace)::ConvertibleToAny, void>' requested here
std::bind(Sink, std::placeholders::_1)(ConvertibleToAny{});
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:128:12:
note: candidate is the implicit move constructor
struct _Head_base<_Idx, _Head, false>
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:128:12:
note: candidate constructor (the implicit copy constructor) has been implicitly
deleted
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:142:7:
note: candidate constructor
_Head_base(__uses_alloc0)
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:271:2:
error: call to constructor of '_Head_base<0UL, (anonymous
namespace)::ConvertibleToAny &&, __empty_not_final<ConvertibleToAny
&&>::value>' is ambiguous
_Base(std::forward<_Head>(_M_head(__in))) { }
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:409:17:
note: in instantiation of member function 'std::_Tuple_impl<0, (anonymous
namespace)::ConvertibleToAny &&>::_Tuple_impl' requested here
constexpr tuple(tuple&&) = default;
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/functional:1354:13:
note: in instantiation of function template specialization
'std::forward_as_tuple<(anonymous namespace)::ConvertibleToAny>' requested here
std::forward_as_tuple(std::forward<_Args>(__args)...),
^
experimental/users/romanp/tuple_bug/tuple-bug.cc:15:41: note: in instantiation
of function template specialization 'std::_Bind<void
(*(std::_Placeholder<1>))(int)>::operator()<(anonymous
namespace)::ConvertibleToAny, void>' requested here
std::bind(Sink, std::placeholders::_1)(ConvertibleToAny{});
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:128:12:
note: candidate is the implicit move constructor
struct _Head_base<_Idx, _Head, false>
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:128:12:
note: candidate constructor (the implicit copy constructor) has been implicitly
deleted
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:142:7:
note: candidate constructor
_Head_base(__uses_alloc0)
^
third_party/crosstool/v17/stable/gcc-x86_64-grtev3-linux-gnu/x86_64-grtev3-linux-gnu/include/c++/4.8.x-google/tuple:868:14:
note: implicit move constructor for 'std::tuple<(anonymous
namespace)::ConvertibleToAny &&>' first required here
{ return tuple<_Elements&&...>(std::forward<_Elements>(__args)...); }
^
This bug appears to be a regression from libstdc++ 4.6, where the code was
accepted.
