[libcxx-commits] [libcxx] [libc++] Fix `unexpected` heterogeneous comparison (PR #115249)

[A. Jiang via libcxx-commits]

https://github.com/frederick-vs-ja created https://github.com/llvm/llvm-project/pull/115249

Currently, libc++ incorrectly rejects heterogeneous comparison of `unexpected`, because the `operator==` is only a hidden friend of `unexpected<_Err>` but not of `unexpected<_Err2>`. We need to call the `error()` member function on `__y`.

Originally discovered by @Quuxplusone. Fixes Quuxplusone/llvm-project#40.

>From 921d491854e226b4a1b007ac7f0e250ea8663322 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34 at live.cn>
Date: Thu, 7 Nov 2024 10:11:33 +0800
Subject: [PATCH] [libc++] Fix `unexpected` heterogeneous comparison

Currently, libc++ incorrectly rejects heterogeneous comparison of
`unexpected`, because the `operator==` is only a hidden friend of
`unexpected<_Err>` but not of `unexpected<_Err2>`. We need to call
`error()` member function on `__y`.
---
 libcxx/include/__expected/unexpected.h              |  2 +-
 .../expected/expected.unexpected/equality.pass.cpp  | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/libcxx/include/__expected/unexpected.h b/libcxx/include/__expected/unexpected.h
index c7fe3c52e43114..cf110bcf69a827 100644
--- a/libcxx/include/__expected/unexpected.h
+++ b/libcxx/include/__expected/unexpected.h
@@ -108,7 +108,7 @@ class unexpected {
 
   template <class _Err2>
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const unexpected& __x, const unexpected<_Err2>& __y) {
-    return __x.__unex_ == __y.__unex_;
+    return __x.__unex_ == __y.error();
   }
 
 private:
diff --git a/libcxx/test/std/utilities/expected/expected.unexpected/equality.pass.cpp b/libcxx/test/std/utilities/expected/expected.unexpected/equality.pass.cpp
index 3c29cf97580460..7098ffc22c5dab 100644
--- a/libcxx/test/std/utilities/expected/expected.unexpected/equality.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.unexpected/equality.pass.cpp
@@ -23,15 +23,28 @@
 struct Error{
   int i;
   friend constexpr bool operator==(const Error&, const Error&) = default;
+  friend constexpr bool operator==(const Error& lhs, int rhs) noexcept {
+    return lhs.i == rhs;
+  }
 };
 
 constexpr bool test() {
   std::unexpected<Error> unex1(Error{2});
   std::unexpected<Error> unex2(Error{3});
   std::unexpected<Error> unex3(Error{2});
+
   assert(unex1 == unex3);
   assert(unex1 != unex2);
   assert(unex2 != unex3);
+
+  std::unexpected<int> unex_i1(1);
+  std::unexpected<int> unex_i2(2);
+
+  assert(unex1 != unex_i1);
+  assert(unex_i1 != unex1);
+  assert(unex1 == unex_i2);
+  assert(unex_i2 == unex1);
+
   return true;
 }