[Clang] Warn about [[noreturn]] on coroutines

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10606,6 +10606,9 @@ def warn_noreturn_function_has_return_expr : Warning<
 def warn_falloff_noreturn_function : Warning<
   "function declared 'noreturn' should not return">,
   InGroup<InvalidNoreturn>;
+def warn_noreturn_coroutine : Warning<
+  "coroutine %0 cannot be declared 'noreturn' as it always returns a coroutine handle">,
+  InGroup<InvalidNoreturn>;
 def err_noreturn_block_has_return_expr : Error<
   "block declared 'noreturn' should not return">;
 def err_carries_dependency_missing_on_first_decl : Error<

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -697,10 +697,12 @@ static void CheckFallThroughForBody(Sema &S, const Decl *D, const Stmt *Body,
       return;
   SourceLocation LBrace = Body->getBeginLoc(), RBrace = Body->getEndLoc();
   auto EmitDiag = [&](SourceLocation Loc, unsigned DiagID) {
-    if (IsCoroutine)
-      S.Diag(Loc, DiagID) << FSI->CoroutinePromise->getType();
-    else
+    if (IsCoroutine) {
+      if (DiagID != 0)
+        S.Diag(Loc, DiagID) << FSI->CoroutinePromise->getType();
+    } else {
       S.Diag(Loc, DiagID);
+    }
   };
 
   // cpu_dispatch functions permit empty function bodies for ICC compatibility.

clang/lib/Sema/SemaCoroutine.cpp

@@ -1176,6 +1176,10 @@ void Sema::CheckCompletedCoroutineBody(FunctionDecl *FD, Stmt *&Body) {
   for (AddrLabelExpr *ALE : Fn->AddrLabels)
     Diag(ALE->getBeginLoc(), diag::err_coro_invalid_addr_of_label);
 
+  // Coroutines always return a handle, so they can't be [[noreturn]].
+  if (FD->isNoReturn())
+    Diag(FD->getLocation(), diag::warn_noreturn_coroutine) << FD;
+
   CoroutineStmtBuilder Builder(*this, *FD, *Fn, Body);
   if (Builder.isInvalid() || !Builder.buildStatements())
     return FD->setInvalidDecl();

clang/lib/Sema/SemaStmt.cpp

@@ -3910,7 +3910,7 @@ StmtResult Sema::BuildReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp,
     FnRetType = FD->getReturnType();
     if (FD->hasAttrs())
       Attrs = &FD->getAttrs();
-    if (FD->isNoReturn())
+    if (FD->isNoReturn() && !getCurFunction()->isCoroutine())
       Diag(ReturnLoc, diag::warn_noreturn_function_has_return_expr) << FD;
     if (FD->isMain() && RetValExp)
       if (isa<CXXBoolLiteralExpr>(RetValExp))

clang/test/SemaCXX/coroutine-noreturn.cpp

@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 %s -std=c++20 -fsyntax-only -Winvalid-noreturn -verify
+
+#include "Inputs/std-coroutine.h"
+
+struct Promise;
+
+struct Awaitable {
+  bool await_ready();
+  void await_suspend(std::coroutine_handle<>);
+  void await_resume();
+};
+
+struct Coro : std::coroutine_handle<> {
+  using promise_type = Promise;
+};
+
+struct Promise {
+  Coro get_return_object();
+  std::suspend_always initial_suspend() noexcept;
+  std::suspend_always final_suspend() noexcept;
+  void return_void();
+  void unhandled_exception();
+};
+
+[[noreturn]] Coro test() { // expected-warning {{coroutine 'test' cannot be declared 'noreturn' as it always returns a coroutine handle}}
+  co_await Awaitable{};
+}
+
+// NO warning here. This could be a regular function returning a `Coro` object.
+[[noreturn]] Coro test2();