[PATCH] D159083: Clang: Don't warn about unused private fields of types declared maybe_unused

[Hans Wennborg via Phabricator via cfe-commits]

hans created this revision.
hans added reviewers: aaron.ballman, shafik.
Herald added a project: All.
hans requested review of this revision.
Herald added a project: clang.

The compiler should not warn on code such as:

  class [[maybe_unused]] MaybeUnusedClass {};
  class C {
    MaybeUnusedClass c;
  };

Patch based on comments on the bug by Shafik and Aaron.

Fixes #61334


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D159083

Files:
  clang/lib/Sema/SemaDeclCXX.cpp
  clang/test/SemaCXX/warn-unused-private-field.cpp


Index: clang/test/SemaCXX/warn-unused-private-field.cpp
===================================================================
--- clang/test/SemaCXX/warn-unused-private-field.cpp
+++ clang/test/SemaCXX/warn-unused-private-field.cpp
@@ -284,3 +284,14 @@
 private:
   int n; // expected-warning{{private field 'n' is not used}}
 };
+
+namespace pr61334 {
+class [[maybe_unused]] MaybeUnusedClass {};
+enum [[maybe_unused]] MaybeUnusedEnum {};
+typedef int MaybeUnusedTypedef [[maybe_unused]];
+class C {
+  MaybeUnusedClass c; // no-warning
+  MaybeUnusedEnum e; // no-warning
+  MaybeUnusedTypedef t; // no-warning
+};
+}
Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -3729,10 +3729,20 @@
     if (!Diags.isIgnored(diag::warn_unused_private_field, FD->getLocation())) {
       // Remember all explicit private FieldDecls that have a name, no side
       // effects and are not part of a dependent type declaration.
+
+      auto DeclHasUnusedAttr = [](const QualType &T) {
+        if (const TagDecl *TD = T->getAsTagDecl())
+          return TD->hasAttr<UnusedAttr>();
+        if (const TypedefType *TDT = T->getAs<TypedefType>())
+          return TDT->getDecl()->hasAttr<UnusedAttr>();
+        return false;
+      };
+
       if (!FD->isImplicit() && FD->getDeclName() &&
           FD->getAccess() == AS_private &&
           !FD->hasAttr<UnusedAttr>() &&
           !FD->getParent()->isDependentContext() &&
+          !DeclHasUnusedAttr(FD->getType()) &&
           !InitializationHasSideEffects(*FD))
         UnusedPrivateFields.insert(FD);
     }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D159083.554260.patch
Type: text/x-patch
Size: 1689 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20230829/d413284e/attachment.bin>