[libcxx-commits] [libcxx] [libc++] Fix `optional<T>::transform` into `optional<T&>` (PR #221506)

[William Tran-Viet via libcxx-commits]

https://github.com/smallp-o-p created https://github.com/llvm/llvm-project/pull/221506

Resolves #220332

- Make `optional<T>` a friend of `optional<T&>`, and add tests.

>From 4b16af031c35bd8686cb2a8786f5c18c9e5f00a8 Mon Sep 17 00:00:00 2001
From: William Tran-Viet <wtranviet at proton.me>
Date: Sat, 5 Sep 2026 20:34:57 -0400
Subject: [PATCH] Fix optional<T>::transform into optional<T&> transform by
 making optional a friend of optional<T&>

---
 libcxx/include/__optional/optional_ref.h      |  3 ++
 .../optional.monadic/transform.pass.cpp       | 33 +++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/libcxx/include/__optional/optional_ref.h b/libcxx/include/__optional/optional_ref.h
index 2fa8c474212e7..802da17f65c53 100644
--- a/libcxx/include/__optional/optional_ref.h
+++ b/libcxx/include/__optional/optional_ref.h
@@ -78,6 +78,9 @@ class optional<_Tp&> : public __optional_ref_iterator_base<_Tp&> {
   static_assert(!is_same_v<remove_cv_t<_Tp>, nullopt_t>, "instantiation of optional with nullopt_t is ill-formed");
 
 private:
+  template <class>
+  friend class optional;
+
   _Tp* __value_ = nullptr;
 
   template <class _Up>
diff --git a/libcxx/test/std/utilities/optional/optional.monadic/transform.pass.cpp b/libcxx/test/std/utilities/optional/optional.monadic/transform.pass.cpp
index 96303c5796f6d..67cd4ba9bea76 100644
--- a/libcxx/test/std/utilities/optional/optional.monadic/transform.pass.cpp
+++ b/libcxx/test/std/utilities/optional/optional.monadic/transform.pass.cpp
@@ -321,6 +321,39 @@ constexpr bool test_ref() {
     assert(&(*o2) == &j);
   }
 
+  // optional<T> -> optional<T&> GH #220332
+  { // &, &&
+    int i = 42;
+    int j = 43;
+    int k = 44;
+
+    std::optional<int> opt{i};
+
+    std::same_as<std::optional<int&>> decltype(auto) o = opt.transform([&](auto) -> int& { return j; });
+    assert(o == 43);
+    assert(&(*o) == &j);
+
+    std::same_as<std::optional<int&>> decltype(auto) o2 = std::move(opt).transform([&](auto) -> int& { return k; });
+    assert(o2 == 44);
+    assert(&(*o2) == &k);
+  }
+
+  { // const&, const&&
+    int i = 42;
+    int j = 43;
+    int k = 44;
+
+    const std::optional<int> opt{i};
+
+    std::same_as<std::optional<int&>> decltype(auto) o = opt.transform([&](auto) -> int& { return j; });
+    assert(o == 43);
+    assert(&(*o) == &j);
+
+    std::same_as<std::optional<int&>> decltype(auto) o2 = std::move(opt).transform([&](auto) -> int& { return k; });
+    assert(o2 == 44);
+    assert(&(*o2) == &k);
+  }
+
   return true;
 }
 #endif