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Improve overload resolution to support P0608 #21

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280bacf
Work on resolve_overload_* functions for P0608 support
Sep 7, 2020
98bd089
Work on resolve_overload_* functions for P0608 support
Sep 8, 2020
c7788fa
Add test case for P0608 support
Sep 8, 2020
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48 changes: 42 additions & 6 deletions include/boost/variant2/variant.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -773,22 +773,60 @@ template<class T0, class T1, class T2, class T3, class T4, class T5, class T6, c

// resolve_overload_*

template<class T> using remove_cv_ref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;

struct narrowing_check_impl
{
template<class T>
static auto check(T (&&)[1]) -> mp11::mp_identity<T>;

template<class T, class U>
using fn = decltype(check<T>({ std::declval<U>() }));
};

template<class T, class U> using narrowing_check = mp11::mp_if<std::is_arithmetic<T>, mp11::mp_identity<T>, mp11::mp_invoke_q<narrowing_check_impl, T, U>>;

template<class... T> struct overload;

template<> struct overload<>
{
void operator()() const;
};

template<class T1, class... T> struct overload<T1, T...>: overload<T...>
template<class T1, class... T> struct overload<T1, T...> : overload<T...>
{
using overload<T...>::operator();
template<class U> auto operator()(T1, U&&) const -> narrowing_check<T1, U>;
};

template<class... T> struct overload<bool, T...> : overload<T...>
{
using overload<T...>::operator();
template<class U> auto operator()(bool, U&&) const -> mp11::mp_if<std::is_same<remove_cv_ref_t<U>, bool>, mp11::mp_identity<bool>>;
};

template<class... T> struct overload<const bool, T...> : overload<T...>
{
using overload<T...>::operator();
mp11::mp_identity<T1> operator()(T1) const;
template<class U> auto operator()(bool, U&&) const -> mp11::mp_if<std::is_same<remove_cv_ref_t<U>, bool>, mp11::mp_identity<const bool>>;
};

template<class... T> struct overload<volatile bool, T...> : overload<T...>
{
using overload<T...>::operator();
template<class U> auto operator()(bool, U&&) const -> mp11::mp_if<std::is_same<remove_cv_ref_t<U>, bool>, mp11::mp_identity<volatile bool>>;
};

template<class... T> struct overload<const volatile bool, T...> : overload<T...>
{
using overload<T...>::operator();
template<class U> auto operator()(bool, U&&) const -> mp11::mp_if<std::is_same<remove_cv_ref_t<U>, bool>, mp11::mp_identity<const volatile bool>>;
};


#if BOOST_WORKAROUND( BOOST_MSVC, < 1930 )

template<class U, class... T> using resolve_overload_type_ = decltype( overload<T...>()(std::declval<U>()) );
template<class U, class... T> using resolve_overload_type_ = decltype( overload<T...>()(std::declval<U>(), std::declval<U>()) );

template<class U, class... T> struct resolve_overload_type_impl: mp11::mp_defer< resolve_overload_type_, U, T... >
{
Expand All @@ -798,7 +836,7 @@ template<class U, class... T> using resolve_overload_type = typename resolve_ove

#else

template<class U, class... T> using resolve_overload_type = typename decltype( overload<T...>()(std::declval<U>()) )::type;
template<class U, class... T> using resolve_overload_type = typename decltype( overload<T...>()(std::declval<U>(), std::declval<U>()) )::type;

#endif

Expand Down Expand Up @@ -1947,8 +1985,6 @@ template<class... T> constexpr bool operator>=( variant<T...> const & v, variant
namespace detail
{

template<class T> using remove_cv_ref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;

template<class... T> variant<T...> const & extract_variant_base_( variant<T...> const & );

#if BOOST_WORKAROUND( BOOST_MSVC, < 1930 )
Expand Down
14 changes: 14 additions & 0 deletions test/variant_convert_construct.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -156,6 +156,20 @@ int main()
BOOST_TEST( holds_alternative<std::string>( v3 ) );
BOOST_TEST_EQ( get<std::string>( v2 ), get<std::string>( v3 ) );
}

{
variant<bool, std::string> v( "s2" );

variant<bool, bool, const char *, std::string> v2( v );

BOOST_TEST( holds_alternative<std::string>( v2 ) );
BOOST_TEST_EQ( get<std::string>( v ), get<std::string>( v2 ) );

variant<bool, bool, const char *, std::string> v3( std::move(v) );

BOOST_TEST( holds_alternative<std::string>( v3 ) );
BOOST_TEST_EQ( get<std::string>( v2 ), get<std::string>( v3 ) );
}

{
variant<X1, X2> v{ X1{1} };
Expand Down