https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99186
Bug ID: 99186
Summary: std::tuple compilation error when elements are
specializations of template class declared with
template < auto E > syntax with E being a enumerator
of a enum
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: huili80 at gmail dot com
Target Milestone: ---
Example code below:
#include <type_traits>
#include <tuple>
enum class E1 {a};
enum class E2 {b,c};
template < auto >
struct S
{
int i;
};
static_assert(not std::is_same_v<S<E1::a>,S<E2::b>>);
struct D
: S<E1::a>, S<E2::b>
{};
int main()
{
D d;
d.S<E1::a>::i = 0;
std::tuple<S<E1::a>,S<E2::b>,S<E2::c>> x;
std::get<0>(x).i = 0;
std::get<S<E2::c>>(x).i = 0;
std::get<S<E2::b>>(x).i = 0; // does not compile
return 0;
}
GCC 10.2 c++17 gives the following error:
In file included from main.cpp:2:
/usr/local/include/c++/10.2.0/tuple: In instantiation of 'constexpr _Tp&
std::get(std::tuple<_UTypes ...>&) [with _Tp = S<E2::b> _Types = {S<E1::a>,
S<E2::b>, S<E2::c>}]':
main.cpp:27:24: required from here
/usr/local/include/c++/10.2.0/tuple:1339:37: error: no matching function for
call to '__get_helper2<S<E2::b> >(std::tuple<S<E1::a>, S<E2::b>, S<E2::c> >&)'
1339 | { return std::__get_helper2<_Tp>(__t); }
| ~~~~~~~~~~~~~~~~~~~~~~~^~~~~
/usr/local/include/c++/10.2.0/tuple:1327:5: note: candidate: 'template<class
_Head, long unsigned int __i, class ... _Tail> constexpr _Head&
std::__get_helper2(std::_Tuple_impl<__i, _Head, _Tail ...>&)'
1327 | __get_helper2(_Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
| ^~~~~~~~~~~~~
/usr/local/include/c++/10.2.0/tuple:1327:5: note: template argument
deduction/substitution failed:
/usr/local/include/c++/10.2.0/tuple:1339:37: note: 'std::_Tuple_impl<__i,
S<E2::b>, _Tail ...>' is an ambiguous base class of 'std::tuple<S<E1::a>,
S<E2::b>, S<E2::c> >'
1339 | { return std::__get_helper2<_Tp>(__t); }
| ~~~~~~~~~~~~~~~~~~~~~~~^~~~~
/usr/local/include/c++/10.2.0/tuple:1332:5: note: candidate: 'template<class
_Head, long unsigned int __i, class ... _Tail> constexpr const _Head&
std::__get_helper2(const std::_Tuple_impl<__i, _Head, _Tail ...>&)'
1332 | __get_helper2(const _Tuple_impl<__i, _Head, _Tail...>& __t)
noexcept
| ^~~~~~~~~~~~~
/usr/local/include/c++/10.2.0/tuple:1332:5: note: template argument
deduction/substitution failed:
/usr/local/include/c++/10.2.0/tuple:1339:37: note: 'const
std::_Tuple_impl<__i, S<E2::b>, _Tail ...>' is an ambiguous base class of
'std::tuple<S<E1::a>, S<E2::b>, S<E2::c> >'
1339 | { return std::__get_helper2<_Tp>(__t); }
| ~~~~~~~~~~~~~~~~~~~~~~~^~~~~
Output:
If E1 and E2 had been non-scoped enums, the code results in the same compiler
error.
It's curious that the error mentions ambiguous base classes in std::tuple's
implementation. Given that std::get<S<E2::c>>(x) compiles fine, I speculated
that the compiler may have incorrectly thought that S<E1::a> and S<E2::b> are
the same type since E1::a and E2::b have the same numeric value (zero).
However, that speculation is proven wrong by other parts of the example code
clearly shows that the compiler does NOT think S<E1::a> and S<E2::b> are the
same type, e.g., the static_assert that S<E1::a> and S<E2::b> are not the same
type, and that even though struct D inherits from both S<E1::a>, S<E2::b>, we
can access its base S<E1::a>'s member variable.
