1 files changed, 126 insertions, 0 deletions
diff --git a/libcxx/test/std/utilities/expected/expected.void/monadic/transform.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/monadic/transform.pass.cpp
new file mode 100644
index 000000000000..ea90006e3199
--- /dev/null
+++ b/libcxx/test/std/utilities/expected/expected.void/monadic/transform.pass.cpp
@@ -0,0 +1,126 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++20
+
+// <expected>
+
+// template<class F> constexpr auto transform_error(F&& f) &;
+// template<class F> constexpr auto transform_error(F&& f) const &;
+// template<class F> constexpr auto transform_error(F&& f) &&;
+// template<class F> constexpr auto transform_error(F&& f) const &&;
+
+#include <expected>
+#include <concepts>
+#include <cassert>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+template <class E, class F>
+concept has_transform =
+    requires(E&& e, F&& f) {
+      { std::forward<E>(e).transform(std::forward<F>(f)) };
+    };
+
+// [LWG 3877] https://cplusplus.github.io/LWG/issue3877, check constraint failing but not compile error inside the function body.
+static_assert(!has_transform<const std::expected<int, std::unique_ptr<int>>&, int()>);
+static_assert(!has_transform<const std::expected<int, std::unique_ptr<int>>&&, int()>);
+
+constexpr void test_val_types() {
+  // Test & overload
+  {
+    auto l = [] -> int { return 1; };
+    std::expected<void, int> v;
+    std::same_as<std::expected<int, int>> decltype(auto) val = v.transform(l);
+    assert(val == 1);
+  }
+
+  // Test const& overload
+  {
+    auto l = [] -> int { return 1; };
+    const std::expected<void, int> v;
+    std::same_as<std::expected<int, int>> decltype(auto) val = v.transform(l);
+    assert(val == 1);
+  }
+
+  // Test && overload
+  {
+    auto l = [] -> int { return 1; };
+    std::expected<void, int> v;
+    std::same_as<std::expected<int, int>> decltype(auto) val = std::move(v).transform(l);
+    assert(val == 1);
+  }
+
+  // Test const&& overload
+  {
+    auto l = [] -> int { return 1; };
+    const std::expected<void, int> v;
+    std::same_as<std::expected<int, int>> decltype(auto) val = std::move(v).transform(l);
+    assert(val == 1);
+  }
+}
+
+constexpr void test_fail() {
+  // Test & overload
+  {
+    auto l = [] -> int {
+      assert(false);
+      return 0;
+    };
+    std::expected<void, int> v(std::unexpected<int>(5));
+    std::same_as<std::expected<int, int>> decltype(auto) val = v.transform(l);
+    assert(val.error() == 5);
+  }
+
+  // Test const& overload
+  {
+    auto l = [] -> int {
+      assert(false);
+      return 0;
+    };
+    const std::expected<void, int> v(std::unexpected<int>(5));
+    std::same_as<std::expected<int, int>> decltype(auto) val = v.transform(l);
+    assert(val.error() == 5);
+  }
+
+  // Test && overload
+  {
+    auto l = [] -> int {
+      assert(false);
+      return 0;
+    };
+    std::expected<void, int> v(std::unexpected<int>(5));
+    std::same_as<std::expected<int, int>> decltype(auto) val = std::move(v).transform(l);
+    assert(val.error() == 5);
+  }
+
+  // Test const&& overload
+  {
+    auto l = [] -> int {
+      assert(false);
+      return 0;
+    };
+    const std::expected<void, int> v(std::unexpected<int>(5));
+    std::same_as<std::expected<int, int>> decltype(auto) val = std::move(v).transform(l);
+    assert(val.error() == 5);
+  }
+}
+
+constexpr bool test() {
+  test_fail();
+  test_val_types();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}