[libc++] Implements concept constructible_from

[Mordante]

Summary

Implements parts of:

• P0898R3 Standard Library Concepts
• P1754 Rename concepts to standard_case for C++20, while we still can

Depends on: D91004

[cjdb]

@CaseyCarter had this to say back in D74291.

There's tons of coverage here for class types and reference types. What about void, function types, pointers, arrays, pointers-to-members? (I don't know about clang/libc++, but the MSVC STL tests for type traits tend to _be_ the tests for compiler intrinsics used to implement those traits, so we consequently cover everything under the sun.)

I'd suggest we apply this here as well.

[Mordante]

In D91986#2412189, @cjdb wrote:

@CaseyCarter had this to say back in D74291.

There's tons of coverage here for class types and reference types. What about void, function types, pointers, arrays, pointers-to-members? (I don't know about clang/libc++, but the MSVC STL tests for type traits tend to _be_ the tests for compiler intrinsics used to implement those traits, so we consequently cover everything under the sun.)

I'd suggest we apply this here as well.

Thanks for the suggestion and I will keep that in mind for other tests. However since this concept is fully specified by the Standard http://eel.is/c++draft/concept.constructible. I prefer to take the approach as suggested in D91004

@ldionne
In general, when writing tests for libc++, one should not assume knowledge of implementation-specific details for the following reasons:

1. The implementation may change in the future, and the tests should still provide us with sufficient confidence in that event.
2. Other standard libraries use our tests, and we can't assume they are implemented the same way.

*However*, this is not implementation-specific in this case, because the Standard (http://eel.is/c++draft/concept.destructible#lib:destructible) specifies that destructible is defined exactly as is_­nothrow_­destructible_­v<T>. It doesn't say "equivalent to is_­nothrow_­destructible_­v<T>" or something like that. I think the best way of testing that would be to throw a bunch of different types and say static_assert(std::destructible<T> == std::is_­nothrow_­destructible_­v<T>).

[cjdb]

In D91986#2414199, @Mordante wrote:

In D91986#2412189, @cjdb wrote:

@CaseyCarter had this to say back in D74291.

There's tons of coverage here for class types and reference types. What about void, function types, pointers, arrays, pointers-to-members? (I don't know about clang/libc++, but the MSVC STL tests for type traits tend to _be_ the tests for compiler intrinsics used to implement those traits, so we consequently cover everything under the sun.)

I'd suggest we apply this here as well.

Thanks for the suggestion and I will keep that in mind for other tests. However since this concept is fully specified by the Standard http://eel.is/c++draft/concept.constructible. I prefer to take the approach as suggested in D91004

@ldionne
In general, when writing tests for libc++, one should not assume knowledge of implementation-specific details for the following reasons:

1. The implementation may change in the future, and the tests should still provide us with sufficient confidence in that event.
2. Other standard libraries use our tests, and we can't assume they are implemented the same way.

*However*, this is not implementation-specific in this case, because the Standard (http://eel.is/c++draft/concept.destructible#lib:destructible) specifies that destructible is defined exactly as is_­nothrow_­destructible_­v<T>. It doesn't say "equivalent to is_­nothrow_­destructible_­v<T>" or something like that. I think the best way of testing that would be to throw a bunch of different types and say static_assert(std::destructible<T> == std::is_­nothrow_­destructible_­v<T>).

@ldionne's comment isn't at odds with @CaseyCarter's. If anything, it's complementing it: void, function types, arrays, and pointer-to-members are all different types, none of which are being tested right now.

[Mordante]

• Rebased.
• Ran clang-format.
• Added more unit tests at @cjdb's suggestion.

[cjdb]

LGTM, pending approval from @EricWF or @ldionne

Thank you for adding the extra tests!

[cjdb]

libcxx/test/std/concepts/concept.constructible/constructible_from.compile.pass.cpp

42–46

Post-LGTM nit: do we really need this one if it's no longer standard?

[Mordante]

Thanks for the review.

libcxx/test/std/concepts/concept.constructible/constructible_from.compile.pass.cpp

42–46

What I mentioned was that ~Throw() throw(int); is no longer supported. However just saw this version is no longer deprecated but has been removed in C++20 http://eel.is/c++draft/diff.cpp17.except.

I'll update the patch. I also used this test in D91004, which I just committed. I'll commit a patch to also remove it in that test.

[Mordante]

The in C++11 deprecated throw() has been removed in C++20. Remove the test using this feature.

libcxx/include/concepts

 template<class _Tp, class _Up>
 concept same_as = __same_as_impl<_Tp, _Up> && __same_as_impl<_Up, _Tp>;
 
 // [concept.destructible]
 
 template<class _Tp>
 concept destructible = _VSTD::is_nothrow_destructible_v<_Tp>;
 
+// [concept.constructible]
+template<class _Tp, class... _Args>
+concept constructible_from =
+    destructible<_Tp> && _VSTD::is_constructible_v<_Tp, _Args...>;
+
 #endif //_LIBCPP_STD_VER > 17 && defined(__cpp_concepts) && __cpp_concepts >= 201811L
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_CONCEPTS

libcxx/test/std/concepts/concept.constructible/constructible_from.compile.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+
+// template<class T, class... Args>
+// concept constructible_from;
+//    destructible<T> && is_constructible_v<T, Args...>;
+
+#include <array>
+#include <concepts>
+#include <memory>
+#include <string>
+#include <type_traits>
+
+struct Empty {};
+
+struct Defaulted {
+  ~Defaulted() = default;
+};
+struct Deleted {
+  ~Deleted() = delete;
+};
+
+struct Noexcept {
+  ~Noexcept() noexcept;
+};
+struct NoexceptTrue {
+  ~NoexceptTrue() noexcept(true);
+};
+struct NoexceptFalse {
+  ~NoexceptFalse() noexcept(false);
+};
+
+struct Protected {
+protected:
+  ~Protected() = default;
+};
+struct Private {
+private:
+  ~Private() = default;
+};
+
+template <class T>
+struct NoexceptDependant {
+  ~NoexceptDependant() noexcept(std::is_same_v<T, int>);
+};
+
+template <class T, class... Args>
+void test() {
+  static_assert(std::constructible_from<T, Args...> ==
+                (std::destructible<T> && std::is_constructible_v<T, Args...>));
+}
+
+void test() {
+  test<bool>();
+  test<bool, bool>();
+
+  test<char>();
+  test<char, char>();
+  test<char, int>();
+
+  test<int>();
+  test<int, int>();
+  test<int, int, int>();
+
+  test<double, int>();
+  test<double, float>();
+  test<double, long double>();
+
+  test<void>();
+  test<void, bool>();
+  test<void, int>();
+
+  test<void*>();
+  test<void*, std::nullptr_t>();
+
+  test<int*>();
+  test<int*, std::nullptr_t>();
+  test<int[], int, int, int>();
+  test<int[1]>();
+  test<int[1], int>();
+  test<int[1], int, int>();
+
+  test<int (*)(int)>();
+  test<int (*)(int), int>();
+  test<int (*)(int), double>();
+  test<int (*)(int), std::nullptr_t>();
+  test<int (*)(int), int (*)(int)>();
+
+  test<void (Empty::*)(const int&)>();
+  test<void (Empty::*)(const int&), std::nullptr_t>();
+  test<void (Empty::*)(const int&) const>();
+  test<void (Empty::*)(const int&) const, void (Empty::*)(const int&)>();
+  test<void (Empty::*)(const int&) volatile>();
+  test<void (Empty::*)(const int&) volatile,
+       void (Empty::*)(const int&) const volatile>();
+  test<void (Empty::*)(const int&) const volatile>();
+  test<void (Empty::*)(const int&) const volatile, double>();
+  test<void (Empty::*)(const int&)&>();
+  test<void (Empty::*)(const int&)&, void (Empty::*)(const int&) &&>();
+  test<void (Empty::*)(const int&) &&>();
+  test<void (Empty::*)(const int&)&&, void (Empty::*)(const int&)>();
+  test<void (Empty::*)(const int&) throw()>();
+  test<void (Empty::*)(const int&) throw(),
+       void(Empty::*)(const int&) noexcept(true)>();
+  test<void (Empty::*)(const int&) noexcept>();
+  test<void (Empty::*)(const int&) noexcept(true)>();
+  test<void (Empty::*)(const int&) noexcept(true),
+       void (Empty::*)(const int&) noexcept(false)>();
+  test<void (Empty::*)(const int&) noexcept(false)>();
+
+  test<int&>();
+  test<int&, int>();
+  test<int&&>();
+  test<int&&, int>();
+
+  test<Empty>();
+
+  test<Defaulted>();
+  test<Deleted>();
+
+  test<NoexceptTrue>();
+  test<NoexceptFalse>();
+  test<Noexcept>();
+
+  test<Protected>();
+  test<Private>();
+
+  test<NoexceptDependant<int> >();
+  test<NoexceptDependant<double> >();
+
+  test<std::string, char*>();
+  test<std::string, const char*>();
+  test<std::string, std::string&>();
+  test<std::string, std::initializer_list<char> >();
+
+  test<std::unique_ptr<int>, std::unique_ptr<int> >();
+  test<std::unique_ptr<int>, std::unique_ptr<int>&>();
+  test<std::unique_ptr<int>, std::unique_ptr<int>&&>();
+
+  test<std::array<int, 1> >();
+  test<std::array<int, 1>, int>();
+  test<std::array<int, 1>, int, int>();
+}