[Clang] Fix -Wunused-private-field false negative with defaulted comparison operators

Fix -Wunused-private-field suppressing warnings when defaulted comparison
operators are declared as friend functions. The warning should only be
suppressed for comparison operators that are class members.

Fixes #116270

Author: whiteio

clang/lib/Sema/SemaDeclCXX.cpp

@@ -7535,7 +7535,7 @@ void Sema::CheckExplicitlyDefaultedFunction(Scope *S, FunctionDecl *FD) {
     return;
   }
 
-  if (DefKind.isComparison())
+  if (DefKind.isComparison() && isa<CXXRecordDecl>(FD->getDeclContext()))
     UnusedPrivateFields.clear();
 
   if (DefKind.isSpecialMember()

clang/test/SemaCXX/warn-unused-private-field.cpp

@@ -40,6 +40,17 @@ class FriendEqDefaultCompareOutOfClass {
 
 bool operator==(const FriendEqDefaultCompareOutOfClass &, const FriendEqDefaultCompareOutOfClass &) = default;
 
+class UnusedConstPrivateField {
+ public:
+  UnusedConstPrivateField() : unused_(0) {}
+ private:
+  const int unused_; // expected-warning{{private field 'unused_' is not used}}
+};
+
+class FriendEqDefaultCompare {
+  friend auto operator==(FriendEqDefaultCompare, FriendEqDefaultCompare) -> bool = default;
+};
+
 #endif
 
 class NotFullyDefined {