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

Author: Nerixyz

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<
+  "coroutines cannot be declared 'noreturn' as they always return 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

@@ -588,10 +588,10 @@ struct CheckFallThroughDiagnostics {
   static CheckFallThroughDiagnostics MakeForCoroutine(const Decl *Func) {
     CheckFallThroughDiagnostics D;
     D.FuncLoc = Func->getLocation();
-    D.diag_MaybeFallThrough_HasNoReturn = 0;
+    D.diag_MaybeFallThrough_HasNoReturn = diag::warn_noreturn_coroutine;
     D.diag_MaybeFallThrough_ReturnsNonVoid =
         diag::warn_maybe_falloff_nonvoid_coroutine;
-    D.diag_AlwaysFallThrough_HasNoReturn = 0;
+    D.diag_AlwaysFallThrough_HasNoReturn = diag::warn_noreturn_coroutine;
     D.diag_AlwaysFallThrough_ReturnsNonVoid =
         diag::warn_falloff_nonvoid_coroutine;
     D.diag_NeverFallThroughOrReturn = 0;

clang/test/SemaCXX/coroutine-noreturn.cpp

@@ -0,0 +1,27 @@
+// 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 {{function 'test' declared 'noreturn' should not return}}
+  co_await Awaitable{};
+} // expected-warning {{coroutines cannot be declared 'noreturn' as they always return a coroutine handle}}