[clang] [Clang] Warn about `[[noreturn]]` on coroutines (PR #127623)

[via cfe-commits]

https://github.com/Nerixyz created https://github.com/llvm/llvm-project/pull/127623

Declaring a coroutine `[[noreturn]]` doesn't make sense, because it will always return its handle. Clang previously crashed when trying to warn about this (diagnostic ID was 0).

This PR adds a warning specific to coroutines. As explained in https://github.com/llvm/llvm-project/issues/127327#issuecomment-2661630422, Clang will show two warnings:

```
main.cpp(41,24): warning: function 'f' declared 'noreturn' should not return [-Winvalid-noreturn]
   41 | [[noreturn]] coroutine f() { co_await awaitable{}; }
      |                        ^
main.cpp(41,52): warning: coroutines cannot be declared 'noreturn' as they always return a coroutine handle [-Winvalid-noreturn]
   41 | [[noreturn]] coroutine f() { co_await awaitable{}; }
      |                                                    ^
2 warnings generated.
```

I'm not sure if that's reasonable, or if the first warning should be omitted.

Fixes #127327.

>From 74d55f99a1197f8141f210e01813a0fdbbeaaeea Mon Sep 17 00:00:00 2001
From: Nerixyz <nerixdev at outlook.de>
Date: Tue, 18 Feb 2025 12:56:05 +0100
Subject: [PATCH] [Clang] Warn about `[[noreturn]]` on coroutines

---
 .../clang/Basic/DiagnosticSemaKinds.td        |  3 +++
 clang/lib/Sema/AnalysisBasedWarnings.cpp      |  4 +--
 clang/test/SemaCXX/coroutine-noreturn.cpp     | 27 +++++++++++++++++++
 3 files changed, 32 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/SemaCXX/coroutine-noreturn.cpp

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index f10af8f5bd6b2..9136fb83ae0b5 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/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<
diff --git a/clang/lib/Sema/AnalysisBasedWarnings.cpp b/clang/lib/Sema/AnalysisBasedWarnings.cpp
index 589869d018657..dc344b60384e3 100644
--- a/clang/lib/Sema/AnalysisBasedWarnings.cpp
+++ b/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;
diff --git a/clang/test/SemaCXX/coroutine-noreturn.cpp b/clang/test/SemaCXX/coroutine-noreturn.cpp
new file mode 100644
index 0000000000000..30454ef0f37cf
--- /dev/null
+++ b/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}}