Use explicit(bool) unconditionally #1301
Description
At our request, C1XX, Clang, and EDG support C++20 explicit(bool) unconditionally - i.e. in C++14/17 modes. (Like if constexpr, C1XX and Clang emit Future Technology warnings that can be suppressed.) This allows us to dramatically simplify pair, tuple, and optional.
We've requested similar support from NVIDIA's CUDA compiler. When this is available, and when MSVC can begin requiring that version of CUDA at a minimum, we should update the STL to remove our fallback codepaths.
Product code examples:
Lines 371 to 380 in 5f3e912
Lines 184 to 207 in 5f3e912
Self-contained test case demonstrating C1XX, Clang, and EDG behavior:
C:\Temp>type explicit.cpp
#include <type_traits>
using namespace std;
#ifdef __clang__
#pragma clang diagnostic ignored "-Wc++20-extensions" // warning: explicit(bool) is a C++20 extension
#else
#pragma warning(disable: 5053) // support for 'explicit(<expr>)' in C++17 and earlier is a vendor extension
#endif
struct No {};
struct Im {};
struct Ex {};
struct A {
A(Im) {}
explicit A(Ex) {}
};
template <typename T, typename Arg>
constexpr bool NotConstructible = !is_constructible_v<T, Arg> && !is_convertible_v<Arg, T>;
template <typename T, typename Arg>
constexpr bool ImplicitlyConstructible = is_constructible_v<T, Arg> && is_convertible_v<Arg, T>;
template <typename T, typename Arg>
constexpr bool ExplicitlyConstructible = is_constructible_v<T, Arg> && !is_convertible_v<Arg, T>;
static_assert(NotConstructible<A, No>, "BOOM NotConstructible");
static_assert(ImplicitlyConstructible<A, Im>, "BOOM ImplicitlyConstructible");
static_assert(ExplicitlyConstructible<A, Ex>, "BOOM ExplicitlyConstructible");
template <typename T1, typename T2> struct OldPair {
template <typename U1, typename U2,
enable_if_t<conjunction_v<is_constructible<T1, const U1&>, is_constructible<T2, const U2&>,
is_convertible<const U1&, T1>, is_convertible<const U2&, T2>>, int> = 0>
OldPair(const OldPair<U1, U2>&) {}
template <typename U1, typename U2,
enable_if_t<conjunction_v<is_constructible<T1, const U1&>, is_constructible<T2, const U2&>,
negation<conjunction<is_convertible<const U1&, T1>, is_convertible<const U2&, T2>>>>, int> = 0>
explicit OldPair(const OldPair<U1, U2>&) {}
};
static_assert(NotConstructible<OldPair<A, A>, const OldPair<No, No>&>, "OldPair BOOM 1");
static_assert(NotConstructible<OldPair<A, A>, const OldPair<No, Im>&>, "OldPair BOOM 2");
static_assert(NotConstructible<OldPair<A, A>, const OldPair<Im, No>&>, "OldPair BOOM 3");
static_assert(ImplicitlyConstructible<OldPair<A, A>, const OldPair<Im, Im>&>, "OldPair BOOM 4");
static_assert(ExplicitlyConstructible<OldPair<A, A>, const OldPair<Im, Ex>&>, "OldPair BOOM 5");
static_assert(ExplicitlyConstructible<OldPair<A, A>, const OldPair<Ex, Im>&>, "OldPair BOOM 6");
static_assert(ExplicitlyConstructible<OldPair<A, A>, const OldPair<Ex, Ex>&>, "OldPair BOOM 7");
template <typename T1, typename T2> struct NewPair {
template <typename U1, typename U2,
enable_if_t<conjunction_v<is_constructible<T1, const U1&>, is_constructible<T2, const U2&>>, int> = 0>
explicit(!is_convertible_v<const U1&, T1> || !is_convertible_v<const U2&, T2>)
NewPair(const NewPair<U1, U2>&) {}
};
static_assert(NotConstructible<NewPair<A, A>, const NewPair<No, No>&>, "NewPair BOOM 1");
static_assert(NotConstructible<NewPair<A, A>, const NewPair<No, Im>&>, "NewPair BOOM 2");
static_assert(NotConstructible<NewPair<A, A>, const NewPair<Im, No>&>, "NewPair BOOM 3");
static_assert(ImplicitlyConstructible<NewPair<A, A>, const NewPair<Im, Im>&>, "NewPair BOOM 4");
static_assert(ExplicitlyConstructible<NewPair<A, A>, const NewPair<Im, Ex>&>, "NewPair BOOM 5");
static_assert(ExplicitlyConstructible<NewPair<A, A>, const NewPair<Ex, Im>&>, "NewPair BOOM 6");
static_assert(ExplicitlyConstructible<NewPair<A, A>, const NewPair<Ex, Ex>&>, "NewPair BOOM 7");
int main() { }C:\Temp>cl /EHsc /nologo /W4 /std:c++14 explicit.cpp
explicit.cpp
C:\Temp>cl /EHsc /nologo /W4 /std:c++14 /c /BE explicit.cpp
explicit.cpp
C:\Temp>clang-cl /EHsc /nologo /W4 /std:c++14 explicit.cpp
C:\Temp>