constexpr template argument weirdness - c++

Question

GCC (5.3) & Clang (3.8) claim that the first line in test is bad, but the second one is fine. MSVC (2015.2) says, both are invalid.
template< typename N, typename T >
void f( N n, T t ) { std::get< n >( t ); }
void test() {
std::get< std::integral_constant< size_t, 0 >() >( std::make_tuple( 123 ) ); // not ok
f( std::integral_constant< size_t, 0 >(), std::make_tuple( 123 ) ); // ok for gcc, clang, but not msvc
}
What, exactly, according to the standard, is the difference? Is this code legal to begin with?
clang error for the first line:
In file included from main.cpp:2:
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.3.0/../../../../include/c++/5.3.0/tuple:874:34: error: no matching function for call to '__get_helper2'
{ return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }
^~~~~~~~~~~~~~~~~~~~~~~
main.cpp:10:10: note: in instantiation of function template specialization 'std::get<std::integral_constant<unsigned long, 0> (), int>' requested here
std::get<std::integral_constant<size_t, 0>()>( std::make_tuple( 123 ) ); // not ok
^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.3.0/../../../../include/c++/5.3.0/tuple:856:5: note: candidate template ignored: could not match '_Tuple_impl' against 'tuple'
__get_helper2(_Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.3.0/../../../../include/c++/5.3.0/tuple:861:5: note: candidate template ignored: could not match '_Tuple_impl' against 'tuple'
__get_helper2(const _Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
^
1 error generated.
gcc error:
In file included from main.cpp:2:0:
/usr/local/include/c++/5.3.0/tuple: In instantiation of 'constexpr _Tp&& std::get(std::tuple<_Elements ...>&&) [with _Tp = std::integral_constant<long unsigned int, 0ul>(); _Types = {int}]':
main.cpp:10:75: required from here
/usr/local/include/c++/5.3.0/tuple:874:57: error: no matching function for call to '__get_helper2(std::tuple<int>&)'
{ return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }
^
/usr/local/include/c++/5.3.0/tuple:856:5: note: candidate: template<class _Head, long unsigned int __i, class ... _Tail> constexpr _Head& std::__get_helper2(std::_Tuple_impl<__i, _Head, _Tail ...>&)
__get_helper2(_Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
^
/usr/local/include/c++/5.3.0/tuple:856:5: note: template argument deduction/substitution failed:
/usr/local/include/c++/5.3.0/tuple:874:57: note: mismatched types 'std::integral_constant<long unsigned int, 0ul>()' and 'int'
{ return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }
^
/usr/local/include/c++/5.3.0/tuple:874:57: note: 'std::tuple<int>' is not derived from 'std::_Tuple_impl<__i, std::integral_constant<long unsigned int, 0ul>(), _Tail ...>'
/usr/local/include/c++/5.3.0/tuple:861:5: note: candidate: template<class _Head, long unsigned int __i, class ... _Tail> constexpr const _Head& std::__get_helper2(const std::_Tuple_impl<__i, _Head, _Tail ...>&)
__get_helper2(const _Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
^
/usr/local/include/c++/5.3.0/tuple:861:5: note: template argument deduction/substitution failed:
/usr/local/include/c++/5.3.0/tuple:874:57: note: mismatched types 'std::integral_constant<long unsigned int, 0ul>()' and 'int'
{ return std::forward<_Tp&&>(std::__get_helper2<_Tp>(__t)); }
^
/usr/local/include/c++/5.3.0/tuple:874:57: note: 'std::tuple<int>' is not derived from 'const std::_Tuple_impl<__i, std::integral_constant<long unsigned int, 0ul>(), _Tail ...>'

Answer 1

tl;dr I think gcc and clang's behavior in both cases is correct.
There is a subtle difference between your two calls. Let me just add an alias to illustrate:
using Zero = std::integral_constant<size_t, 0>;
auto tuple = std::make_tuple(123);
When you write:
std::get<Zero()>(tuple);
Zero() is a type. It is a nullary function returning a Zero. As such, you're calling the version of std::get that takes a type: std::get<T>(). Since there is no element in tuple that has type Zero(), this is an error.
On the other hand, when you write:
Zero n;
std::get<n>(tuple);
n is not a type - it is only ever a value. Since std::integral_constant has a constexpr operator size_t(), that one gets used and you end up calling std::get<I>(), which does what you'd expect.
The same could be accomplished by simply using brace initialization as well:
std::get<Zero{}>(tuple);
since Zero{} is definitely not a type.