unqualified call to `get<T>` with a non-`boost::optional` type should not match

[Kojoley]

boost::get<T>(boost::optional<T> ...) overload matches for a non-boost::optional type when boost namespace is associated with argument's type of unqualified call to get<T>.

template <typename U>
struct X {};

template <typename T, typename U>
inline int get(X<U> const&) { return 0; }

namespace boost {
 struct any_type_in_boost_namespace {};
}

#include <boost/optional.hpp>

class tag;

int main()
{
 get<tag>(X<boost::any_type_in_boost_namespace>{});
}

https://godbolt.org/z/14d476

The example obviously should compile, but instead it tries to instantiate boost::optional<tag> and fails the compilation.

[akrzemi1]

Thanks for reporting this. I should be able to look at it in a couple of days.