[libcxx-commits] [libcxx] [libc++] Implement ranges::iota (PR #68494)

[Konstantin Varlamov via libcxx-commits]

================
@@ -0,0 +1,218 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Testing std::ranges::iota
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <numeric>
+#include <utility>
+
+#include "almost_satisfies_types.h"
+#include "test_iterators.h"
+#include "test_macros.h"
+
+//
+// Testing constraints
+//
+
+// Concepts to check different overloads of std::ranges::iota
+template <class Iter = int*, class Sent = int*, class Value = int>
+concept HasIotaIter = requires(Iter&& iter, Sent&& sent, Value&& val) {
+  std::ranges::iota(std::forward<Iter>(iter), std::forward<Sent>(sent), std::forward<Value>(val));
+};
+
+template <class Range, class Value = int>
+concept HasIotaRange =
+    requires(Range&& range, Value&& val) { std::ranges::iota(std::forward<Range>(range), std::forward<Value>(val)); };
+
+// Test constraints of the iterator/sentinel overload
+// ==================================================
+static_assert(HasIotaIter<int*, int*, int>);
+
+// !input_or_output_iterator<O>
+static_assert(!HasIotaIter<InputIteratorNotInputOrOutputIterator>);
+
+// !sentinel_for<S, O>
+static_assert(!HasIotaIter<int*, SentinelForNotSemiregular>);
+static_assert(!HasIotaIter<int*, SentinelForNotWeaklyEqualityComparableWith>);
+
+// !weakly_incrementable<T>
+static_assert(!HasIotaIter<int*, int*, WeaklyIncrementableNotMovable>);
+
+// !indirectly writable <O, T>
+static_assert(!HasIotaIter<OutputIteratorNotIndirectlyWritable, int*, int>);
+
+// Test constraints for the range overload
+// =======================================
+static_assert(HasIotaRange<UncheckedRange<int*>, int>);
+
+// !weakly_incrementable<T>
+static_assert(!HasIotaRange<UncheckedRange<int*>, WeaklyIncrementableNotMovable>);
+
+// !ranges::output_range<const _Tp&>
+static_assert(!HasIotaRange<UncheckedRange<int*>, OutputIteratorNotIndirectlyWritable>);
+
+//
+// Testing results
+//
+
+struct DangerousCopyAssign {
+  int val;
+  using difference_type = int;
+
+  constexpr explicit DangerousCopyAssign(int v) : val(v) {}
+
+  // Needed in postfix
+  constexpr DangerousCopyAssign(DangerousCopyAssign const& other) { this->val = other.val; }
+
+  /*
+  This class has a "mischievous" non-const overload of copy-assignment
+  operator that modifies the object being assigned from. `ranges::iota`
+  should not be invoking this overload thanks to the `std::as_const` in its
+  implementation. If for some reason it does invoke it, the values written
+  by ranges::iota will increment by 2 rather than 1.
+  */
+  constexpr DangerousCopyAssign& operator=(DangerousCopyAssign& a) {
+    ++a.val;
+    this->val = a.val;
+    return *this;
+  }
+
+  // safe copy assignment std::as_const inside ranges::iota should ensure this
+  // overload gets called
+  constexpr DangerousCopyAssign& operator=(DangerousCopyAssign const& a) {
+    this->val = a.val;
+    return *this;
+  }
+
+  constexpr bool operator==(DangerousCopyAssign const& rhs) { return this->val == rhs.val; }
+
+  // prefix
+  constexpr DangerousCopyAssign& operator++() {
+    ++(this->val);
+    return *this;
+  }
+
+  // postfix
+  constexpr DangerousCopyAssign operator++(int) {
+    auto tmp = *this;
+    ++this->val;
+    return tmp;
+  }
+};
+
+template <class Iter, class Sent, std::size_t N>
+constexpr void test_result(std::array<int, N> input, int starting_value, std::array<int, N> const expected) {
+  { // (iterator, sentinel) overload
+    auto in_begin = Iter(input.data());
+    auto in_end   = Sent(Iter(input.data() + input.size()));
+    std::same_as<std::ranges::out_value_result<Iter, int>> decltype(auto) result =
+        std::ranges::iota(std::move(in_begin), std::move(in_end), starting_value);
+    assert(result.out == in_end);
+    assert(result.value == starting_value + static_cast<int>(N));
+    assert(std::ranges::equal(input, expected));
+  }
+
+  { // (range) overload
+    // in the range overload adds the additional constraint that it must be an output range
+    // so skip this for the input iterators we test
+    auto in_begin = Iter(input.data());
+    auto in_end   = Sent(Iter(input.data() + input.size()));
+    auto range    = std::ranges::subrange(std::move(in_begin), std::move(in_end));
+
+    std::same_as<std::ranges::out_value_result<Iter, int>> decltype(auto) result =
+        std::ranges::iota(range, starting_value);
+    assert(result.out == in_end);
+    assert(result.value == starting_value + static_cast<int>(N));
+    assert(std::ranges::equal(input, expected));
+  }
+}
+
+template <class Iter, class Sent = sentinel_wrapper<Iter>>
+constexpr void test_results() {
+  // Empty
+  test_result<Iter, Sent, 0>({}, 0, {});
+  // 1-element sequence
+  test_result<Iter, Sent, 1>({1}, 0, {0});
+  // Longer sequence
+  test_result<Iter, Sent, 5>({1, 2, 3, 4, 5}, 0, {0, 1, 2, 3, 4});
+}
+
+constexpr void test_user_defined_type() {
+  // Simple non-fundamental type
+  struct UserDefinedType {
+    int val;
+    using difference_type = int;
+
+    constexpr explicit UserDefinedType(int v) : val(v) {}
+    constexpr UserDefinedType(UserDefinedType const& other) { this->val = other.val; }
+    constexpr UserDefinedType& operator=(UserDefinedType const& a) {
+      this->val = a.val;
+      return *this;
+    }
+
+    // prefix
+    constexpr UserDefinedType& operator++() {
+      ++(this->val);
+      return *this;
+    }
+
+    // postfix
+    constexpr UserDefinedType operator++(int) {
+      auto tmp = *this;
+      ++this->val;
+      return tmp;
+    }
+  };
+
+  // Setup
+  using A                                 = UserDefinedType;
+  std::array<UserDefinedType, 5> a        = {A{0}, A{0}, A{0}, A{0}, A{0}};
+  std::array<UserDefinedType, 5> expected = {A{0}, A{1}, A{2}, A{3}, A{4}};
+
+  // Fill with values
+  std::ranges::iota(a, A{0});
+  auto proj_val = [](UserDefinedType const& el) { return el.val; };
+
+  // Check
+  assert(std::ranges::equal(a, expected, std::ranges::equal_to{}, proj_val, proj_val));
+}
+
+constexpr void test_danderous_copy_assign() {
+  using A = DangerousCopyAssign;
+
+  // If the dangerous non-const copy assignment is called, the final values in
+  // aa should increment by 2 rather than 1.
+  std::array<A, 3> aa       = {A{0}, A{0}, A{0}};
+  std::array<A, 3> expected = {A{0}, A{1}, A{2}};
+  std::ranges::iota(aa, A{0});
+  auto proj_val = [](DangerousCopyAssign const& el) { return el.val; };
+  assert(std::ranges::equal(aa, expected, std::ranges::equal_to{}, proj_val, proj_val));
+}
+
+constexpr bool test_results() {
+  // Tests on fundamental types
+  types::for_each(types::cpp17_input_iterator_list<int*>{}, []<class Iter> { test_results< Iter>(); });
+  test_results<cpp17_output_iterator<int*>>();
+  test_results<cpp20_output_iterator<int*>>();
+  test_results<int*, sized_sentinel<int*>>();
+
+  // Tests on non-fundamental types
+  test_user_defined_type();
+  test_danderous_copy_assign();
+  return true;
+}
+
+int main(int, char**) {
+  static_assert(test_results());
----------------
var-const wrote:

We need to test both in `constexpr` mode and at runtime, so this should be:
```
test_results();
static_assert(test_results());
```
It's a pattern we follow in general -- it's always possible for code to have different behavior between the two modes, whether due to `if consteval`, compiler bugs, etc.


https://github.com/llvm/llvm-project/pull/68494