[clang] a4fbc09 - Clang: Don't warn about unused private fields of types declared maybe_unused

[Hans Wennborg via cfe-commits]

Author: Hans Wennborg
Date: 2023-08-30T09:06:39+02:00
New Revision: a4fbc091846206fb6f5bc36115d65075764b51ea

URL: https://github.com/llvm/llvm-project/commit/a4fbc091846206fb6f5bc36115d65075764b51ea
DIFF: https://github.com/llvm/llvm-project/commit/a4fbc091846206fb6f5bc36115d65075764b51ea.diff

LOG: Clang: Don't warn about unused private fields of types declared maybe_unused

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

Differential revision: https://reviews.llvm.org/D159083

Added: 
    

Modified: 
    clang/docs/ReleaseNotes.rst
    clang/lib/Sema/SemaDeclCXX.cpp
    clang/test/SemaCXX/warn-unused-private-field.cpp

Removed: 
    


################################################################################
diff  --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 1bd295c68b83b7..1e273c1b7ce46a 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -202,6 +202,9 @@ Bug Fixes in This Version
 - Clang now emits an error if it is not possible to deduce array size for a
   variable with incomplete array type.
   (`#37257 <https://github.com/llvm/llvm-project/issues/37257>`_)
+- Clang's ``-Wunused-private-field`` no longer warns on fields whose type is
+  declared with ``[[maybe_unused]]``.
+  (`#61334 <https://github.com/llvm/llvm-project/issues/61334>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

diff  --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index a7ab0948d01bf3..2b2b7391f88fd5 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -3729,10 +3729,20 @@ Sema::ActOnCXXMemberDeclarator(Scope *S, AccessSpecifier AS, Declarator &D,
     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);
     }

diff  --git a/clang/test/SemaCXX/warn-unused-private-field.cpp b/clang/test/SemaCXX/warn-unused-private-field.cpp
index e67603eaceae66..1128eacc309d9f 100644
--- a/clang/test/SemaCXX/warn-unused-private-field.cpp
+++ b/clang/test/SemaCXX/warn-unused-private-field.cpp
@@ -284,3 +284,14 @@ class defaulted_special_member {
 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
+};
+}