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

[via cfe-commits]

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@@ -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}}
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cor3ntin wrote:

I'm not sure the warning is useful at all when the function is an override (especially in code that is old enough not to have used override from the get-go). And there is no downside to marking them virtual - nor do I think it's confusing.

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