[clang] Warn about virtual methods in `final` classes (PR #131188)

Thu Mar 13 11:48:01 PDT 2025

llvmbot wrote:




@llvm/pr-subscribers-clang

Author: Devon Loehr (DKLoehr)

<details>
<summary>Changes</summary>

There's never any point to adding a `virtual` specifier to methods in a `final` class, since the class can't be subclassed. This adds a warning when we notice this happening, as suggested in #131108.

We don't currently implement the second part of the suggestion, to warn on `virtual` methods which are never overridden anywhere. Although it's feasible to do this for things with internal linkage (so we can check at the end of the TU), it's more complicated to implement and it's not clear it's worth the effort.

I tested the warning by compiling chromium and clang itself. Chromium resulted in [277 warnings across 109 files](https://github.com/user-attachments/files/19234889/warnings-chromium.txt), while clang had [38 warnings across 29 files](https://github.com/user-attachments/files/19234888/warnings-clang.txt). I inspected a subset of the warning sites manually, and they all seemed legitimate.

This warning is very easy to fix (just remove the `virtual` specifier) and I haven't seen any false positives, so it's suitable for on-by-default. However, I've currently made it off-by-default because it fires at several places in the repo. I plan to submit a followup PR fixing those places and enabling the warning by default.

---
Full diff: https://github.com/llvm/llvm-project/pull/131188.diff


5 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+3) 
- (modified) clang/include/clang/Basic/DiagnosticGroups.td (+11) 
- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+3) 
- (modified) clang/lib/Sema/SemaDeclCXX.cpp (+5) 
- (added) clang/test/SemaCXX/unnecessary-virtual-specifier.cpp (+27) 


``````````diff

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8989124611e66..15858631c17b7 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -247,6 +247,9 @@ Improvements to Clang's diagnostics
 - The ``-Wsign-compare`` warning now treats expressions with bitwise not(~) and minus(-) as signed integers 
   except for the case where the operand is an unsigned integer
   and throws warning if they are compared with unsigned integers (##18878).
+- The ``-Wunnecessary-virtual-specifier`` warning has been added to warn about
+  methods which are marked as virtual inside a ``final`` class, and hence can
+  never be overridden.
 
 - Improve the diagnostics for chained comparisons to report actual expressions and operators (#GH129069).
 
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index fac80fb4009aa..e785e84205192 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -372,6 +372,17 @@ def CXX11WarnInconsistentOverrideMethod :
 def CXX11WarnSuggestOverrideDestructor : DiagGroup<"suggest-destructor-override">;
 def CXX11WarnSuggestOverride : DiagGroup<"suggest-override">;
 
+def WarnUnnecessaryVirtualSpecifier : DiagGroup<"unnecessary-virtual-specifier"> {
+  code Documentation = [{
+Warns when a ``final`` class contains a virtual method (including virtual
+destructors). Since ``final`` classes cannot be subclassed, their methods
+cannot be overridden, and hence the ``virtual`` specifier is useless.
+
+The warning also detects virtual methods in classes whose destructor is
+``final``, for the same reason.
+  }];
+}
+
 // Original name of this warning in Clang
 def : DiagGroup<"c++0x-narrowing", [CXX11Narrowing]>;
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 8e6e6e892cdd7..ed31c2d225073 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2706,6 +2706,9 @@ def warn_final_dtor_non_final_class : Warning<
   InGroup<FinalDtorNonFinalClass>;
 def note_final_dtor_non_final_class_silence : Note<
   "mark %0 as '%select{final|sealed}1' to silence this warning">;
+def warn_unnecessary_virtual_specifier : Warning<
+  "virtual method %0 is inside a 'final' class and can never be overridden">,
+  InGroup<WarnUnnecessaryVirtualSpecifier>, DefaultIgnore;
 
 // C++11 attributes
 def err_repeat_attribute : Error<"%0 attribute cannot be repeated">;
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index dd779ee377309..1b2e494956d4b 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7193,10 +7193,15 @@ void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
         // class without overriding any.
         if (!M->isStatic())
           DiagnoseHiddenVirtualMethods(M);
+
         if (M->hasAttr<OverrideAttr>())
           HasMethodWithOverrideControl = true;
         else if (M->size_overridden_methods() > 0)
           HasOverridingMethodWithoutOverrideControl = true;
+
+        // See if a method is marked as virtual inside of a final class.
+        if (M->isVirtualAsWritten() && Record->isEffectivelyFinal())
+          Diag(M->getLocation(), diag::warn_unnecessary_virtual_specifier) << M;
       }
 
       if (!isa<CXXDestructorDecl>(M))
diff --git a/clang/test/SemaCXX/unnecessary-virtual-specifier.cpp b/clang/test/SemaCXX/unnecessary-virtual-specifier.cpp
new file mode 100644
index 0000000000000..00f696bfa0d4f
--- /dev/null
+++ b/clang/test/SemaCXX/unnecessary-virtual-specifier.cpp
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wunnecessary-virtual-specifier %s
+
+struct Foo final {
+  Foo() = default;
+	virtual ~Foo() = default;                      // expected-warning {{virtual method}}
+	virtual Foo& operator=(Foo& other) = default;  // expected-warning {{virtual method}}
+	virtual Foo& operator=(Foo&& other) = default; // expected-warning {{virtual method}}
+	void f();
+  virtual void f(int);                           // expected-warning {{virtual method}}
+  int g(int x) { return x; };
+  virtual int g(bool);                           // expected-warning {{virtual method}}
+  static int s();
+};
+
+struct BarBase {
+  virtual ~BarBase() = delete;
+	virtual void virt() {}
+	virtual int virt(int);
+  int nonvirt();
+};
+
+struct Bar final : BarBase {
+  ~Bar() override = delete;
+	void virt() override {};
+	virtual int virt(int) override;                // expected-warning {{virtual method}}
+  int nonvirt();
+};

``````````

</details>


https://github.com/llvm/llvm-project/pull/131188
