[clang] Use the correct namespace for looking up matching operator!= (PR #68922)

[Utkarsh Saxena via cfe-commits]

https://github.com/usx95 updated https://github.com/llvm/llvm-project/pull/68922

>From 8aa439a97a56ef80bfc9ccc90a9f093680e455f5 Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Sun, 22 Oct 2023 11:11:53 +0200
Subject: [PATCH] rebase...

---
 clang/docs/ReleaseNotes.rst                   |  4 +
 clang/lib/Sema/SemaOverload.cpp               | 19 ++---
 .../over.match.oper/p3-2a.cpp                 | 78 +++++++++++++++++++
 3 files changed, 89 insertions(+), 12 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3a9a67f7451c092..cef857244387e8b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -85,6 +85,10 @@ C/C++ Language Potentially Breaking Changes
     ``__has_c_attribute(warn_unused_result)``,          202003, 0
     ``__has_c_attribute(gnu::warn_unused_result)``,     202003, 1
 
+- Fixed a bug in finding matching `operator!=` while adding reversed `operator==` as
+  outlined in "The Equality Operator You Are Looking For" (`P2468 <http://wg21.link/p2468r2>`_).
+  Fixes (`#68901: <https://github.com/llvm/llvm-project/issues/68901>`_).
+
 C++ Specific Potentially Breaking Changes
 -----------------------------------------
 - The name mangling rules for function templates has been changed to take into
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index d57a7ad8f46859a..75e4184c8aa6cb8 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -960,18 +960,13 @@ static bool shouldAddReversedEqEq(Sema &S, SourceLocation OpLoc,
     return true;
   }
   // Otherwise the search scope is the namespace scope of which F is a member.
-  LookupResult NonMembers(S, NotEqOp, OpLoc,
-                          Sema::LookupNameKind::LookupOperatorName);
-  S.LookupName(NonMembers,
-               S.getScopeForContext(EqFD->getEnclosingNamespaceContext()));
-  NonMembers.suppressAccessDiagnostics();
-  for (NamedDecl *Op : NonMembers) {
-    auto *FD = Op->getAsFunction();
-    if(auto* UD = dyn_cast<UsingShadowDecl>(Op))
-      FD = UD->getUnderlyingDecl()->getAsFunction();
-    if (FunctionsCorrespond(S.Context, EqFD, FD) &&
-        declaresSameEntity(cast<Decl>(EqFD->getDeclContext()),
-                           cast<Decl>(Op->getDeclContext())))
+  for (NamedDecl *Op : EqFD->getEnclosingNamespaceContext()->lookup(NotEqOp)) {
+    auto *NotEqFD = Op->getAsFunction();
+    if (auto *UD = dyn_cast<UsingShadowDecl>(Op))
+      NotEqFD = UD->getUnderlyingDecl()->getAsFunction();
+    if (FunctionsCorrespond(S.Context, EqFD, NotEqFD) &&
+        declaresSameEntity(cast<Decl>(EqFD->getEnclosingNamespaceContext()),
+                           cast<Decl>(Op->getLexicalDeclContext())))
       return false;
   }
   return true;
diff --git a/clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp b/clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp
index 5727d506fe5e61d..9dc5ee8db565341 100644
--- a/clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp
+++ b/clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp
@@ -374,6 +374,84 @@ bool fine(A a, B b) { return a == b; } // Ok. Found a matching operator!=.
 }
 }
 
+
+namespace ADL_GH68901{
+namespace A {
+struct S {};
+bool operator==(S, int); // expected-note {{no known conversion from 'int' to 'S' for 1st argument}}
+bool operator!=(S, int);
+} // namespace A
+bool a = 0 == A::S(); // expected-error {{invalid operands to binary expression ('int' and 'A::S')}}
+
+namespace B {
+struct Derived {};
+struct Base : Derived {};
+
+bool operator==(Derived& a, Base& b);
+bool operator!=(Derived& a, Base& b);
+}  // namespace B
+
+void foo() {
+  // B::Base{} == B::Base{};
+  B::Base a,b;
+  bool v = a == b;
+}
+
+namespace ns {
+template <class T>
+struct A {
+};
+
+template <class T>
+struct B : A<T> {
+};
+
+template <class T> bool operator == (B<T>, A<T>); // expected-note {{candidate template ignored: could not match 'B' against 'A'}}
+template <class T> bool operator != (B<T>, A<T>);
+}
+
+void test() {
+    ns::A<int> a;
+    ns::B<int> b;
+    a == b; // expected-error {{invalid operands to binary expression}}
+}
+
+
+} //namespace ADL_GH68901
+
+namespace function_scope_operator_eqeq {
+// For non-members, we always lookup for matching operator!= in the namespace scope of
+// operator== (and not in the scope of operator==).
+struct X { operator int(); };
+namespace test1{
+bool h(X x) {
+  bool operator==(X, int); // expected-note {{reversed}}
+  return x == x; // expected-warning {{ambiguous}}
+}
+
+bool g(X x) {
+  bool operator==(X, int); // expected-note {{reversed}}
+  bool operator!=(X, int);
+  return x == x;  // expected-warning {{ambiguous}}
+}
+} // namespace test1
+
+namespace test2 {
+bool operator!=(X, int);
+
+bool h(X x) {
+  bool operator==(X, int);
+  return x == x;
+}
+
+bool i(X x) {
+  bool operator==(X, int);
+  bool operator!=(X, int);
+  return x == x;
+}
+} // namespace test2
+} // namespace function_scope_operator_eqeq
+
 namespace non_member_template_2 {
 struct P {};
 template<class S>