Use the correct namespace for looking up matching operator!= (#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

Author: usx95

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

clang/lib/Sema/SemaOverload.cpp

@@ -962,18 +962,13 @@ 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) && S.isVisible(NotEqFD) &&
+ declaresSameEntity(cast<Decl>(EqFD->getEnclosingNamespaceContext()),
+ cast<Decl>(Op->getLexicalDeclContext())))
  return false;
  }
  return true;

clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p2468R2.cppm

@@ -0,0 +1,24 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/A.cppm -emit-module-interface -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -I%t %t/p2468r2.cpp -verify
+
+//--- A.cppm
+module;
+export module A;
+export {
+namespace NS {
+struct S {};
+bool operator==(S, int);
+} // namespace NS
+}
+
+namespace NS { bool operator!=(S, int); } // Not visible.
+
+
+//--- p2468r2.cpp
+// expected-no-diagnostics
+import A;
+bool x = 0 == NS::S(); // Ok. operator!= from module A is not visible.

clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp

@@ -374,6 +374,109 @@ bool fine(A a, B b) { return a == b; } // Ok. Found a matching operator!=.
 }
 }
 
+
+namespace ADL_GH68901{
+namespace test1 {
+namespace A {
+struct S {};
+bool operator==(S, int); // expected-note {{no known conversion from 'int' to 'S' for 1st argument}}
+bool a = 0 == A::S(); // Ok. Operator!= not visible.
+bool operator!=(S, int);
+} // namespace A
+bool a = 0 == A::S(); // expected-error {{invalid operands to binary expression ('int' and 'A::S')}}
+} // namespace test1
+
+namespace test2 {
+namespace B {
+struct Derived {};
+struct Base : Derived {};
+
+bool operator==(Derived& a, Base& b);
+bool operator!=(Derived& a, Base& b);
+} // namespace B
+
+bool foo() {
+ B::Base a,b;
+ return a == b;
+}
+} // namespace test2
+
+
+namespace template_ {
+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 test3
+
+namespace using_not_eq {
+namespace A {
+struct S {};
+namespace B {
+bool operator!=(S, int);
+}
+bool operator==(S, int); // expected-note {{candidate}}
+using B::operator!=;
+} // namespace A
+bool a = 0 == A::S(); // expected-error {{invalid operands to binary expression}}
+} // namespace reversed_lookup_not_like_ADL
+
+namespace using_eqeq {
+namespace A {
+struct S {};
+namespace B {
+bool operator==(S, int); // expected-note {{candidate}}
+bool operator!=(S, int);
+}
+using B::operator==;
+} // namespace A
+bool a = 0 == A::S(); // 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>