[libc++] Fix unexpected heterogeneous comparison

[frederick-vs-ja]

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#40.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: A. Jiang (frederick-vs-ja)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/115249.diff

2 Files Affected:

• (modified) libcxx/include/__expected/unexpected.h (+1-1)
• (modified) libcxx/test/std/utilities/expected/expected.unexpected/equality.pass.cpp (+13)

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;
 }
 

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