Unusual compiler message in gcc-8.2 - c++

Question

The following piece of code works as expected but gives an odd looking message during compilation. Not sure what to make of it, but just trying to understand what GCC is trying to convey. Is it an error or just a GCC oddity? This is not happening in gcc-10.2
#include <algorithm>
#include <array>
#include <stdexcept>
#include <optional>
template < typename T >
struct Optional : public std::optional < T >
{
using impl_t = std::optional < T >;
using impl_t::impl_t;
constexpr inline friend bool operator == (Optional const & lhs, Optional const & rhs)
{
return static_cast < impl_t const & >(lhs) == static_cast < impl_t const & >(rhs);
}
template < typename U, typename = std::enable_if_t < std::is_convertible_v < U, T > > >
constexpr inline friend bool operator == (const Optional& lhs, U const & rhs)
{
return lhs.has_value() && static_cast < impl_t const & >(lhs) == rhs;
}
};
[[maybe_unused]]
auto triggers_unusual_compile_warning()
{
constexpr std::array < Optional < int32_t >, 4 > c{{1, 2, 3, std::nullopt}};
if (std::count(c.begin(), c.end(), std::nullopt) != 1)
throw std::runtime_error("WTH!!!");
};
int main()
{
triggers_unusual_compile_warning();
}
That small program triggers the following message
/opt/compiler-explorer/gcc-8.2.0/include/c++/8.2.0/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Iter_equals_val<_Value>::operator()(_Iterator) [with _Iterator = const Optional<int>*; _Value = const std::nullopt_t]':
/opt/compiler-explorer/gcc-8.2.0/include/c++/8.2.0/bits/stl_algo.h:3194:12: required from 'typename std::iterator_traits<_Iterator>::difference_type std::__count_if(_InputIterator, _InputIterator, _Predicate) [with _InputIterator = const Optional<int>*; _Predicate = __gnu_cxx::__ops::_Iter_equals_val<const std::nullopt_t>; typename std::iterator_traits<_Iterator>::difference_type = long int]'
/opt/compiler-explorer/gcc-8.2.0/include/c++/8.2.0/bits/stl_algo.h:4082:29: required from 'typename std::iterator_traits<_Iterator>::difference_type std::count(_IIter, _IIter, const _Tp&) [with _IIter = const Optional<int>*; _Tp = std::nullopt_t; typename std::iterator_traits<_Iterator>::difference_type = long int]'
<source>:29:52: required from here
/opt/compiler-explorer/gcc-8.2.0/include/c++/8.2.0/optional:1371:5: note: candidate 1: 'constexpr bool std::operator==(const std::optional<_Tp>&, std::nullopt_t) [with _Tp = int]'
operator==(const optional<_Tp>& __lhs, nullopt_t) noexcept
^~~~~~~~
<source>:12:34: note: candidate 2: 'constexpr bool operator==(const Optional<int>&, const Optional<int>&)'
constexpr inline friend bool operator == (Optional const & lhs, Optional const & rhs)
^~~~~~~~
ASM generation compiler returned: 0
In file included from <source>:4:
/opt/compiler-explorer/gcc-8.2.0/include/c++/8.2.0/bits/predefined_ops.h: In instantiation of 'bool __gnu_cxx::__ops::_Iter_equals_val<_Value>::operator()(_Iterator) [with _Iterator = const Optional<int>*; _Value = const std::nullopt_t]':
/opt/compiler-explorer/gcc-8.2.0/include/c++/8.2.0/bits/stl_algo.h:3194:12: required from 'typename std::iterator_traits<_Iterator>::difference_type std::__count_if(_InputIterator, _InputIterator, _Predicate) [with _InputIterator = const Optional<int>*; _Predicate = __gnu_cxx::__ops::_Iter_equals_val<const std::nullopt_t>; typename std::iterator_traits<_Iterator>::difference_type = long int]'
/opt/compiler-explorer/gcc-8.2.0/include/c++/8.2.0/bits/stl_algo.h:4082:29: required from 'typename std::iterator_traits<_Iterator>::difference_type std::count(_IIter, _IIter, const _Tp&) [with _IIter = const Optional<int>*; _Tp = std::nullopt_t; typename std::iterator_traits<_Iterator>::difference_type = long int]'
<source>:29:52: required from here
/opt/compiler-explorer/gcc-8.2.0/include/c++/8.2.0/optional:1371:5: note: candidate 1: 'constexpr bool std::operator==(const std::optional<_Tp>&, std::nullopt_t) [with _Tp = int]'
operator==(const optional<_Tp>& __lhs, nullopt_t) noexcept
^~~~~~~~
<source>:12:34: note: candidate 2: 'constexpr bool operator==(const Optional<int>&, const Optional<int>&)'
constexpr inline friend bool operator == (Optional const & lhs, Optional const & rhs)
^~~~~~~~
Execution build compiler returned: 0

Answer 1

The problem with the code is that the equality operator is ambiguous. A std::nullopt can be converted to a std::optional.
So, by instantiating a operator== when you call std::count, the compiler cannot choose between the operator== from the library and your operator== declared as
constexpr inline friend bool operator == (Optional const & lhs, Optional const & rhs)
How exactly to solve this problem depends on the details you have in mind. For example, if you add an explicit comparison with std::nullopt_t
constexpr inline friend bool operator == (Optional const & lhs, std::nullopt_t const & rhs)
{
return static_cast < impl_t const & >(lhs) == static_cast < impl_t const & >(rhs);
}
you force the compiler to choose your more-specific implementation, and the error disappears.
But of course, a good solution depends on what you have in mind.