[Clang] suppress -Wmissing-noreturn for virtual methods with throw-only bodies (#167523)

Fixes #167247

--- 

This PR addresses a case where Clang emitted `-Wmissing-noreturn` for
virtual methods whose body consists of a `throw` expression

```cpp
struct Base {
  virtual void foo() {
    throw std::runtime_error("error");
  }
};
```

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -434,6 +434,8 @@ Improvements to Clang's diagnostics
   or continue (#GH166013)
 - Clang now emits a diagnostic in case `vector_size` or `ext_vector_type`
   attributes are used with a negative size (#GH165463).
+- Clang no longer emits ``-Wmissing-noreturn`` for virtual methods where
+  the function body consists of a `throw` expression (#GH167247).
 
 Improvements to Clang's time-trace
 ----------------------------------

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1966,8 +1966,16 @@ static bool isKnownToAlwaysThrow(const FunctionDecl *FD) {
   if (const auto *EWC = dyn_cast<ExprWithCleanups>(OnlyStmt)) {
     OnlyStmt = EWC->getSubExpr();
   }
-  // Check if the only statement is a throw expression.
-  return isa<CXXThrowExpr>(OnlyStmt);
+
+  if (isa<CXXThrowExpr>(OnlyStmt)) {
+    const auto *MD = dyn_cast<CXXMethodDecl>(FD);
+    if (MD && MD->isVirtual()) {
+      const auto *RD = MD->getParent();
+      return MD->hasAttr<FinalAttr>() || (RD && RD->isEffectivelyFinal());
+    }
+    return true;
+  }
+  return false;
 }
 
 void clang::inferNoReturnAttr(Sema &S, const Decl *D) {

clang/test/SemaCXX/wmissing-noreturn-suggestion.cpp

@@ -53,3 +53,26 @@ int gnu_throws() {
 int cxx11_throws() {
     throw 0;
 }
+
+namespace GH167247 {
+struct S1 {
+  virtual ~S1() = default;
+  virtual void m() {
+    throw std::runtime_error("This method always throws");
+  }
+};
+
+struct S2 {
+  virtual ~S2() = default;
+
+  virtual void m() final { // expected-warning {{function 'm' could be declared with attribute 'noreturn'}}
+    throw std::runtime_error("This method always throws");
+  }
+};
+
+struct S3 final : S1 {
+  void m() { // expected-warning {{function 'm' could be declared with attribute 'noreturn'}}
+    throw std::runtime_error("This method always throws");
+  }
+};
+}