[Clang] Only ignore special methods for unused private fields in BuildFieldReferenceExpr (#116965)

The original code assumed that only special methods might be defined as
defaulted. Since C++20 comparison operators might be defaulted too, and
we *do* want to consider those as using the fields of the class.

Fixes: #116961

Author: Maetveis

clang/docs/ReleaseNotes.rst

@@ -591,6 +591,8 @@ Improvements to Clang's diagnostics
 - For an rvalue reference bound to a temporary struct with an integer member, Clang will detect constant integer overflow
   in the initializer for the integer member (#GH46755).
 
+- Fixed a false negative ``-Wunused-private-field`` diagnostic when a defaulted comparison operator is defined out of class (#GH116961).
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaExprMember.cpp

@@ -1874,8 +1874,16 @@ Sema::BuildFieldReferenceExpr(Expr *BaseExpr, bool IsArrow,
           Context.getAttributedType(attr::NoDeref, MemberType, MemberType);
   }
 
-  auto *CurMethod = dyn_cast<CXXMethodDecl>(CurContext);
-  if (!(CurMethod && CurMethod->isDefaulted()))
+  auto isDefaultedSpecialMember = [this](const DeclContext *Ctx) {
+    auto *Method = dyn_cast<CXXMethodDecl>(CurContext);
+    if (!Method || !Method->isDefaulted())
+      return false;
+
+    return getDefaultedFunctionKind(Method).isSpecialMember();
+  };
+
+  // Implicit special members should not mark fields as used.
+  if (!isDefaultedSpecialMember(CurContext))
     UnusedPrivateFields.remove(Field);
 
   ExprResult Base = PerformObjectMemberConversion(BaseExpr, SS.getScopeRep(),

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

@@ -20,6 +20,26 @@ class SpaceShipDefaultCompare {
   int operator<=>(const SpaceShipDefaultCompare &) const = default;
 };
 
+class EqDefaultCompareOutOfClass {
+  int used; // no warning, the compiler generated AST for the comparison operator
+            // references the fields of the class, and this should be considered
+            // a use.
+            // This test case is needed because clang does not emit the body
+            // of the defaulted operator when it is defined in-class until it
+            // finds a call to it. `-Wunused-private-field` is suppressed in
+            // a different way in that case.
+  bool operator==(const EqDefaultCompareOutOfClass &) const;
+};
+
+bool EqDefaultCompareOutOfClass::operator==(const EqDefaultCompareOutOfClass &) const = default;
+
+class FriendEqDefaultCompareOutOfClass {
+  int used; // no warning, same reasoning just tested via a friend declaration.
+  friend bool operator==(const FriendEqDefaultCompareOutOfClass &, const FriendEqDefaultCompareOutOfClass &);
+};
+
+bool operator==(const FriendEqDefaultCompareOutOfClass &, const FriendEqDefaultCompareOutOfClass &) = default;
+
 #endif
 
 class NotFullyDefined {