[clang] Warning for non-final classes with final destructors

Marking a class' destructor final prevents the class from being inherited from. However, it is a subtle and awkward way to express that at best, and unintended at worst. It may also generate worse code (in other compilers) than marking the class itself final. For these reasons, this revision adds a warning for nonfinal classes with final destructors, with a note to suggest marking the class final to silence the warning.

See https://reviews.llvm.org/D66621 for more background.

Patch by logan-5 (Logan Smith)

Differential Revision: https://reviews.llvm.org/D66711


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370594 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/SemaDeclCXX.cpp

@@ -6236,6 +6236,19 @@ void Sema::CheckCompletedCXXClass(CXXRecordDecl *Record) {
     }
   }
 
+  // Warn if the class has a final destructor but is not itself marked final.
+  if (!Record->hasAttr<FinalAttr>()) {
+    if (const CXXDestructorDecl *dtor = Record->getDestructor()) {
+      if (const FinalAttr *FA = dtor->getAttr<FinalAttr>()) {
+        Diag(FA->getLocation(), diag::warn_final_dtor_non_final_class)
+          << FA->isSpelledAsSealed();
+        Diag(Record->getLocation(), diag::note_final_dtor_non_final_class_silence)
+          << Context.getRecordType(Record)
+          << FA->isSpelledAsSealed();
+      }
+    }
+  }
+
   // See if trivial_abi has to be dropped.
   if (Record->hasAttr<TrivialABIAttr>())
     checkIllFormedTrivialABIStruct(*Record);

test/SemaCXX/MicrosoftExtensions.cpp

@@ -440,6 +440,11 @@ struct SealedType sealed : SomeBase {
 // expected-error@+1 {{base 'SealedType' is marked 'sealed'}}
 struct InheritFromSealed : SealedType {};
 
+class SealedDestructor { // expected-note {{mark 'SealedDestructor' as 'sealed' to silence this warning}}
+    // expected-warning@+1 {{'sealed' keyword is a Microsoft extension}}
+    virtual ~SealedDestructor() sealed; // expected-warning {{class with destructor marked 'sealed' cannot be inherited from}}
+};
+
 void AfterClassBody() {
   // expected-warning@+1 {{attribute 'deprecated' is ignored, place it after "struct" to apply attribute to type declaration}}
   struct D {} __declspec(deprecated);

test/SemaCXX/warn-final-dtor-non-final-class.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify %s -Wfinal-dtor-non-final-class
+
+class A {
+    ~A();
+};
+
+class B { // expected-note {{mark 'B' as 'final' to silence this warning}}
+    virtual ~B() final; // expected-warning {{class with destructor marked 'final' cannot be inherited from}}
+};
+
+class C final {
+    virtual ~C() final;
+};