[clang][c++20] Fix false warning for unused private fields when a class has only defaulted comparison operators.

[predator5047]

Summary

Fixes bug 50263

When "unused-private-field" flag is on if you have a struct with private members and only defaulted comparison operators clang will warn about unused private fields.

If you where to write the comparison operators by hand no warning is produced.

This is a bug since defaulting a comparison operator uses all private members .

The fix is simple, in CheckExplicitlyDefaultedFunction just clear the list of unused private fields if the defaulted function is a comparison function.

[predator5047]

ping

[hans]

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

7

The test case feels a little strange to me, I mean the member variable is still pretty unused even though I suppose the comparison function technically uses it.. At least the test case in the bug report had a constructor which set the value. Perhaps with that it makes more sense.

[predator5047]

Updated the test case so it is clearer.

[predator5047]

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

7

Thanks for your feedback I updated the test case based on your comments.

[predator5047]

@hans Can you commit on my behalf?
Email: Alexandru Octavian Butiu alexandru.octavian.butiu at gmail.com

[hans]

In D102186#2832900, @predator5047 wrote:

@hans Can you commit on my behalf?
Email: Alexandru Octavian Butiu alexandru.octavian.butiu at gmail.com

Sure! Pushed as https://github.com/llvm/llvm-project/commit/78d404a11dd33c8349fd9b6ef5876d523c457f0e

clang/lib/Sema/SemaDeclCXX.cpp

void Sema::CheckExplicitlyDefaultedFunction(Scope *S, FunctionDecl *FD) {
   assert(FD->isExplicitlyDefaulted() && "not explicitly-defaulted");
 
   DefaultedFunctionKind DefKind = getDefaultedFunctionKind(FD);
   if (!DefKind) {
     assert(FD->getDeclContext()->isDependentContext());
     return;
   }
 
+  if (DefKind.isComparison())
+    UnusedPrivateFields.clear();
+
   if (DefKind.isSpecialMember()
           ? CheckExplicitlyDefaultedSpecialMember(cast<CXXMethodDecl>(FD),
                                                   DefKind.asSpecialMember())
           : CheckExplicitlyDefaultedComparison(S, FD, DefKind.asComparison()))
     FD->setInvalidDecl();
 }
 
 bool Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,

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

 // RUN: %clang_cc1 -fsyntax-only -Wunused-private-field -Wused-but-marked-unused -Wno-uninitialized -verify -std=c++11 %s
 // RUN: %clang_cc1 -fsyntax-only -Wunused-private-field -Wused-but-marked-unused -Wno-uninitialized -verify -std=c++17 %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-private-field -Wused-but-marked-unused -Wno-uninitialized -verify -std=c++20 %s
+
+#if __cplusplus >= 202002L
+
+class EqDefaultCompare {
+  int used;
+
+public:
+  EqDefaultCompare(int x) : used(x) {}
+  bool operator==(const EqDefaultCompare &) const = default;
+};
+
+class SpaceShipDefaultCompare {
+  int used;
+
+public:
+  SpaceShipDefaultCompare(int x) : used(x) {}
+  int operator<=>(const SpaceShipDefaultCompare &) const = default;
+};
+
+#endif
 
 class NotFullyDefined {
  public:
   NotFullyDefined();
  private:
   int y;
 };