[clang] [Sema] Suggest [[noreturn]] for void functions that always throw (#146234)

Implements #146223.

Author: snarang181

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1979,6 +1979,10 @@ void clang::inferNoReturnAttr(Sema &S, const Decl *D) {
   if (!FD->hasAttr<NoReturnAttr>() && !FD->hasAttr<InferredNoReturnAttr>() &&
       isKnownToAlwaysThrow(FD)) {
     NonConstFD->addAttr(InferredNoReturnAttr::CreateImplicit(S.Context));
+
+    // Emit a diagnostic suggesting the function being marked [[noreturn]].
+    S.Diag(FD->getLocation(), diag::warn_suggest_noreturn_function)
+        << /*isFunction=*/0 << FD;
   }
 }
 

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

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -fsyntax-only -fcxx-exceptions -fexceptions -Wreturn-type -Wmissing-noreturn -verify %s
+
+namespace std {
+  class string {
+  public:
+    string(const char*);
+  };
+  class runtime_error {
+  public:
+    runtime_error(const string&);
+  };
+}
+
+// This function always throws. Suggest [[noreturn]].
+void throwError(const std::string& msg) { // expected-warning {{function 'throwError' could be declared with attribute 'noreturn'}}
+  throw std::runtime_error(msg);
+}
+
+// The non-void caller should not warn about missing return.
+int ensureZero(int i) {
+  if (i == 0) return 0;
+  throwError("ERROR"); // no-warning
+}