[Clang] Only ignore special methods for unused private fields in BuildFieldReferenceExpr

[Maetveis]

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

[llvmbot]

@llvm/pr-subscribers-clang

Author: Mészáros Gergely (Maetveis)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/116965.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaExprMember.cpp (+10-2)
• (modified) clang/test/SemaCXX/warn-unused-private-field.cpp (+14)

diff --git a/clang/lib/Sema/SemaExprMember.cpp b/clang/lib/Sema/SemaExprMember.cpp
index c32df607692813..46821dc8746b2a 100644
--- a/clang/lib/Sema/SemaExprMember.cpp
+++ b/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(),
diff --git a/clang/test/SemaCXX/warn-unused-private-field.cpp b/clang/test/SemaCXX/warn-unused-private-field.cpp
index 1128eacc309d9f..9913dbaafb7a57 100644
--- a/clang/test/SemaCXX/warn-unused-private-field.cpp
+++ b/clang/test/SemaCXX/warn-unused-private-field.cpp
@@ -20,6 +20,20 @@ class SpaceShipDefaultCompare {
   int operator<=>(const SpaceShipDefaultCompare &) const = default;
 };
 
+class EqDefaultCompareOutOfClass {
+  int used;
+  bool operator==(const EqDefaultCompareOutOfClass &) const;
+};
+
+bool EqDefaultCompareOutOfClass::operator==(const EqDefaultCompareOutOfClass &) const = default;
+
+class FriendEqDefaultCompareOutOfClass {
+  int used;
+  friend bool operator==(const FriendEqDefaultCompareOutOfClass &, const FriendEqDefaultCompareOutOfClass &);
+};
+
+bool operator==(const FriendEqDefaultCompareOutOfClass &, const FriendEqDefaultCompareOutOfClass &) = default;
+
 #endif
 
 class NotFullyDefined {

[erichkeane]

Needs a release note, else LGTM.

[erichkeane]

Please explain a touch better with 'no warning'? Something like, "Test to ensure that the defaulted operator results in this being seen as 'used' despite there not being code to do so."?