[libcxx][test] Verify customization point object properties

[jloser]

Summary

Add test for various customization point object properties as defined by
the Standard. Test various CPOs from <ranges>, <iterator>,
<concepts>, etc.

The test is mostly from https://reviews.llvm.org/D107036 and split up
into this - thanks Arthur!

[jloser]

Remove unused <array> include.

[jloser]

Abandoning - did not mean to open. Instead, force updated https://reviews.llvm.org/D115588 to address review feedback. Sorry about that.

libcxx/test/std/library/description/conventions/customization.point.object/cpo.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, libcpp-has-no-incomplete-ranges
+
+// [customization.point.object]
+// [range.adaptor.object] "A range adaptor object is a customization point object..."
+
+#include <compare>
+#include <iterator>
+#include <ranges>
+#include <utility>
+
+// Test for basic properties of C++20 16.3.3.3.6 [customization.point.object].
+template <class CPO, class... Args>
+constexpr bool test(CPO& o, Args&&...) {
+  static_assert(std::is_class_v<CPO>);
+  static_assert(std::is_trivial_v<CPO>);
+
+  auto p = o;
+  using T = decltype(p);
+
+  // The type of a customization point object, ignoring cv-qualifiers, shall model semiregular.
+  static_assert(std::semiregular<T>);
+
+  // The type T of a customization point object, ignoring cv-qualifiers, shall model...
+  static_assert(std::invocable<T&, Args...>);
+  static_assert(std::invocable<const T&, Args...>);
+  static_assert(std::invocable<T, Args...>);
+  static_assert(std::invocable<const T, Args...>);
+
+  return true;
+}
+
+int main(int, char**)
+{
+    int a[10];
+    //int arrays[10][10];
+    //std::pair<int, int> pairs[10];
+
+    // [concept.swappable]
+    static_assert(test(std::ranges::swap, a, a));
+
+    // [iterator.cust]
+    static_assert(test(std::ranges::iter_move, a + 0));
+    static_assert(test(std::ranges::iter_swap, a + 0, a + 1));
+
+    // [cmp.alg]
+    static_assert(test(std::partial_order, 1, 2));
+    static_assert(test(std::strong_order, 1, 2));
+    static_assert(test(std::weak_order, 1, 2));
+    //static_assert(test(std::compare_partial_order_fallback, 1, 2));
+    //static_assert(test(std::compare_strong_order_fallback, 1, 2));
+    //static_assert(test(std::compare_weak_order_fallback, 1, 2));
+
+    // [range.access]
+    static_assert(test(std::ranges::begin, a));
+    static_assert(test(std::ranges::end, a));
+    static_assert(test(std::ranges::cbegin, a));
+    //static_assert(test(std::ranges::cdata, a));
+    static_assert(test(std::ranges::cend, a));
+    //static_assert(test(std::ranges::crbegin, a));
+    //static_assert(test(std::ranges::crend, a));
+    static_assert(test(std::ranges::data, a));
+    static_assert(test(std::ranges::empty, a));
+    //static_assert(test(std::ranges::rbegin, a));
+    //static_assert(test(std::ranges::rend, a));
+    static_assert(test(std::ranges::size, a));
+    static_assert(test(std::ranges::ssize, a));
+
+
+    // [range.factories]
+    // views::empty<T> is not a CPO
+    static_assert(test(std::views::iota, 1));
+    static_assert(test(std::views::iota, 1, 10));
+    //static_assert(test(std::views::istream<int>, 1);
+    //static_assert(test(std::views::single, 4));
+
+    // [range.adaptors]
+    static_assert(test(std::views::all, a));
+    static_assert(test(std::views::common, a));
+    static_assert(test(std::views::counted, a, 10));
+    //static_assert(test(std::views::drop, a, 10));
+    //static_assert(test(std::views::drop_while, a, [](int x){ return x < 10; }));
+    //static_assert(test(std::views::elements<0>, pairs));
+    //static_assert(test(std::views::filter, a, [](int x){ return x < 10; }));
+    //static_assert(test(std::views::join, arrays));
+    static_assert(test(std::views::reverse, a));
+    //static_assert(test(std::views::split, a, 4));
+    //static_assert(test(std::views::take, a, 10));
+    //static_assert(test(std::views::take_while, a, [](int x){ return x < 10; }));
+    static_assert(test(std::views::transform, a, [](int x){ return x + 1; }));
+
+    return 0;
+}