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Fix is_convertible_without_narrowing for corner cases #57

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Merged
saki7 merged 1 commit into from
Aug 9, 2026
Merged

Fix is_convertible_without_narrowing for corner cases #57

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107 changes: 92 additions & 15 deletions include/iris/type_traits.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -223,39 +223,116 @@ template<class T>
constexpr bool is_trivially_swappable_v = is_trivially_swappable<T>::value;


// P0870R7: is_convertible_without_narrowing
// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p0870r7.html
namespace detail {

// Workaround for MSVC 2026's Intellisense
template<class To, class From>
inline constexpr bool reference_converts_from_temporary_workaround_v =
#if defined(__cpp_lib_reference_from_temporary)
std::reference_converts_from_temporary_v<To, From>;
#elif defined(__has_builtin) && __has_builtin(__reference_converts_from_temporary)
__reference_converts_from_temporary(To, From);
#else
true;
#endif

// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p0870r8.html

template<class From, class To>
struct is_convertible_without_narrowing_impl
struct is_convertible_without_narrowing_array_check
: std::false_type
{};

// void to void "conversion" is valid since `is_convertible` is defined
// as "returning `From` is valid for function whose return type is `To`?"
template<>
struct is_convertible_without_narrowing_impl<void, void>
template<class From, class To>
requires requires (From&& x) {
{ std::type_identity_t<To[]>{std::forward<From>(x)} } -> std::same_as<To[1]>;
}
struct is_convertible_without_narrowing_array_check<From, To>
: std::true_type
{};

// ----------------------------------------------

// Array of reference cannot be formed, handle special case.
template<class From, class To>
struct is_convertible_without_narrowing_dispatch
: is_convertible_without_narrowing_array_check<From, To>
{
static_assert(!std::is_reference_v<To>);
};

template<class From, class To>
using is_never_narrowing_family = std::disjunction<
std::is_same<std::remove_cvref_t<From>, std::remove_cvref_t<To>>,
std::is_base_of<std::remove_cvref_t<From>, std::remove_cvref_t<To>>,
std::is_function<std::remove_cvref_t<To>>,
std::is_array<std::remove_cvref_t<To>>
>;
template<class From, class To>
requires
std::is_reference_v<To> &&
is_never_narrowing_family<From, To>::value
struct is_convertible_without_narrowing_dispatch<From, To>
: std::true_type
{};

template<class From, class To>
requires
requires (From&& x) {
{ std::type_identity_t<To[]>{std::forward<From>(x)} } -> std::same_as<To[1]>;
}
std::is_reference_v<To> &&
(!is_never_narrowing_family<From, To>::value) &&
(!reference_converts_from_temporary_workaround_v<To, From>)
struct is_convertible_without_narrowing_dispatch<From, To>
: std::true_type
{};

template<class From, class To>
requires
std::is_reference_v<To> &&
(!is_never_narrowing_family<From, To>::value) &&
reference_converts_from_temporary_workaround_v<To, From>
struct is_convertible_without_narrowing_dispatch<From, To>
: is_convertible_without_narrowing_array_check<
From,
std::remove_reference_t<To> // temporary type is copy-list-initialized
>
{};

// ----------------------------------------------

template<class From, class To>
struct is_convertible_without_narrowing_impl
: is_convertible_without_narrowing_dispatch<From, To>
{};

// Corner case mentioned on the paper: void
// cv variants are already handled via `std::is_convertible`.
template<class From, class To>
requires std::is_void_v<To>
struct is_convertible_without_narrowing_impl<From, To>
: std::true_type
{};

// DR11: Converting from T* to bool should be considered narrowing
// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1957r2.html
//
// This is already applied to all major vendors, but some implementations
// disagree with `std::nullptr_t`. Note that `std::nullptr_t` is NOT a
// pointer type, so it cannot be checked with `std::is_pointer`.
template<class From, class To>
requires std::is_null_pointer_v<From> && std::same_as<std::remove_cvref_t<To>, bool>
struct is_convertible_without_narrowing_impl<From, To> : std::false_type
{};

} // namespace detail

template<class From, class To>
struct is_convertible_without_narrowing
: std::conjunction<
std::is_convertible<From, To>,
detail::is_convertible_without_narrowing_impl<From, To>
>
struct is_convertible_without_narrowing : std::false_type
{};

template<class From, class To>
requires std::is_convertible_v<From, To>
struct is_convertible_without_narrowing<From, To>
: detail::is_convertible_without_narrowing_impl<From, To>
{};

template<class From, class To>
Expand Down
118 changes: 76 additions & 42 deletions test/type_traits.cpp
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -19,20 +19,6 @@ struct n_tuple;
template<int... Ns>
struct n_list;

struct convertible_from_int
{
convertible_from_int(int);
};

struct not_convertible_from_int {};

struct explicit_from_int
{
explicit explicit_from_int() = default;
explicit explicit_from_int(int);
explicit_from_int& operator=(int) { return *this; }
};

enum class scoped_enum {};
enum unscoped_enum {};
enum class scoped_enum_uint8 : unsigned char {};
Expand All @@ -51,25 +37,8 @@ struct explicit_conversion_op
explicit operator int() const;
};

struct abstract_class
{
virtual void f() = 0;
};

struct multi_arg_implicit
{
multi_arg_implicit(int, double);
};

struct has_initializer_list_ctor
{
has_initializer_list_ctor(std::initializer_list<int>);
};

struct member_ptr_test
{
int member;
void func();
};

} // anonymous
Expand Down Expand Up @@ -266,8 +235,26 @@ TEST_CASE("is_convertible_without_narrowing: pointer types")
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<void (member_ptr_test::*)(), int>);
}

TEST_CASE("is_convertible_without_narrowing: class types with implicit ctor")
TEST_CASE("is_convertible_without_narrowing: constexpr caveat")
{
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<int, float>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<int const, float>);

// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p0870r8.html#ch5.9
// The paper wants true in this case.
// TODO: GCC/Clang and MSVC(2026) disagree; unfixable
//STATIC_CHECK(iris::is_convertible_without_narrowing_v<std::integral_constant<int, 42>, float>);
}

TEST_CASE("is_convertible_without_narrowing: class types with conversion")
{
struct convertible_from_int
{
convertible_from_int(int);
};

struct not_convertible_from_int {};

// Implicit converting constructor
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int, convertible_from_int>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<convertible_from_int, int>);
Expand All @@ -276,15 +263,31 @@ TEST_CASE("is_convertible_without_narrowing: class types with implicit ctor")
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<int, not_convertible_from_int>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<not_convertible_from_int, int>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<int, std::string>);
}

TEST_CASE("is_convertible_without_narrowing: class types with explicit ctor")
{
// ------------------------------------------

struct explicit_from_int
{
explicit explicit_from_int() = default;
explicit explicit_from_int(int) {}
explicit_from_int& operator=(int) { return *this; }
};

// Explicit ctor: is_convertible is false, so trait is false
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<int, explicit_from_int>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<explicit_from_int, int>);

// ------------------------------------------

struct convertible_to_double
{
operator double();
};
STATIC_CHECK(!iris::is_convertible_without_narrowing<double, float>::value);
STATIC_CHECK(!iris::is_convertible_without_narrowing<convertible_to_double, float>::value);
}


TEST_CASE("is_convertible_without_narrowing: class types with conversion operator")
{
// Implicit conversion operator
Expand All @@ -304,7 +307,7 @@ TEST_CASE("is_convertible_without_narrowing: class types inheritance")

// Derived& to base: implicitly convertible
STATIC_CHECK(iris::is_convertible_without_narrowing_v<derived&, base>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<const derived&, base>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<derived const&, base>);
}

TEST_CASE("is_convertible_without_narrowing: aggregate types")
Expand Down Expand Up @@ -350,6 +353,9 @@ TEST_CASE("is_convertible_without_narrowing: void")
{
// void to void: is_convertible_v<void, void> is true per the standard.
STATIC_CHECK(iris::is_convertible_without_narrowing_v<void, void>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<void, void const>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<void const, void const>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<void const, void>);

// void to/from anything else: not convertible
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<void, int>);
Expand All @@ -360,12 +366,12 @@ TEST_CASE("is_convertible_without_narrowing: void")
TEST_CASE("is_convertible_without_narrowing: cv-qualified types")
{
// const arithmetic: same narrowing rules
STATIC_CHECK(iris::is_convertible_without_narrowing_v<const int, int>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int, const int>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<const int, long long>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<const long long, int>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<volatile int, int>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<const volatile int, int>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int const, int>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int, int const>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int const, long long>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<long long const, int>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int volatile, int>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int const volatile, int>);
}

TEST_CASE("is_convertible_without_narrowing: reference types as From")
Expand All @@ -382,6 +388,34 @@ TEST_CASE("is_convertible_without_narrowing: reference types as From")
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<long long&&, int>);
}

TEST_CASE("is_convertible_without_narrowing: reference types as To")
{
STATIC_CHECK(std::is_convertible_v<int&, int&>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int&, int&>);

STATIC_CHECK(std::is_convertible_v<int&, int const&>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<int&, int const&>);

STATIC_CHECK(!std::is_convertible_v<int const&, int&>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<int const&, int&>);

STATIC_CHECK(!std::is_convertible_v<int, int&>);
STATIC_CHECK(!iris::is_convertible_without_narrowing_v<int, int&>);

STATIC_CHECK(std::is_convertible_v<float, double const&>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<float, double const&>);

// --------------------------------------------------

using F = void();

STATIC_CHECK(std::is_convertible_v<F&, F&>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<F&, F&>);

STATIC_CHECK(std::is_convertible_v<F, F&>);
STATIC_CHECK(iris::is_convertible_without_narrowing_v<F, F&>);
}

TEST_CASE("is_convertible_without_narrowing: function pointers")
{
using fp = void(*)();
Expand Down
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