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

[Utkarsh Saxena via cfe-commits]

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

`S.getScopeForContext` determins the **active** scope associated with the given `declContext`. 
This fails to find the matching `operator!=` if candidate `operator==` was found via ADL since that scope is not active.

Instead, just directly lookup using the namespace decl of `operator==`

Fixes #68901

>From 0c71b6bdd557ff4b9ad9a4ec4f0919e3460596b0 Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Thu, 12 Oct 2023 21:35:52 +0200
Subject: [PATCH] Find opertor!= in the correct namespace

---
 clang/lib/Sema/SemaOverload.cpp               |  7 +-----
 .../over.match.oper/p3-2a.cpp                 | 24 +++++++++++++++++++
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index ce78994e6553814..e1e0b4a888e49fa 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -960,12 +960,7 @@ 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) {
+  for (NamedDecl *Op : EqFD->getEnclosingNamespaceContext()->lookup(NotEqOp)) {
     auto *FD = Op->getAsFunction();
     if(auto* UD = dyn_cast<UsingShadowDecl>(Op))
       FD = UD->getUnderlyingDecl()->getAsFunction();
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 5c6804eb7726b5f..7142ed22ddb22ca 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
@@ -324,6 +324,30 @@ bool x = X() == X(); // expected-warning {{ambiguous}}
 }
 } // namespace P2468R2
 
+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 ADL_GH68901
+
+
 #else // NO_ERRORS
 
 namespace problem_cases {