[Coroutines] Special handle __builtin_coro_resume for final_suspend nothrow check

[lxfind]

Summary

In https://reviews.llvm.org/D82029 we added the conformance check that the expression co_await promise.final_suspend() should not potentially throw.
As part of this expression, in cases when the await_suspend() method of the final suspend awaiter returns a handle, builtin_coro_resume could be called on the handle to immediately resume that coroutine.
builtin_coro_resume is not declared with noexcept and it shouldn't. We need to special check this case here.

[lxfind]

Address lint

[lxfind]

rebase

[lewissbaker]

LGTM

clang/lib/Sema/SemaCoroutine.cpp

621

... it does not throw _into_ the coroutine that just suspended, but rather throws back out from whoever called coroutine_handle::resume().

clang/lib/Sema/SemaCoroutine.cpp

 /// (including constructor call) is declared noexcept. Also any value returned
 /// from the call has a noexcept destructor.
 static void checkNoThrow(Sema &S, const Stmt *E,
                          llvm::SmallPtrSetImpl<const Decl *> &ThrowingDecls) {
   auto checkDeclNoexcept = [&](const Decl *D, bool IsDtor = false) {
     // In the case of dtor, the call to dtor is implicit and hence we should
     // pass nullptr to canCalleeThrow.
     if (Sema::canCalleeThrow(S, IsDtor ? nullptr : cast<Expr>(E), D)) {
+      if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
+        // co_await promise.final_suspend() could end up calling
+        // __builtin_coro_resume for symmetric transfer if await_suspend()
+        // returns a handle. In that case, even __builtin_coro_resume is not
+        // declared as noexcept and may throw, it does not throw _into_ the
+        // coroutine that just suspended, but rather throws back out from
+        // whoever called coroutine_handle::resume(), hence we claim that
+        // logically it does not throw.
+        if (FD->getBuiltinID() == Builtin::BI__builtin_coro_resume)
+          return;
+      }
       if (ThrowingDecls.empty()) {
         // First time seeing an error, emit the error message.
         S.Diag(cast<FunctionDecl>(S.CurContext)->getLocation(),
                diag::err_coroutine_promise_final_suspend_requires_nothrow);
       }
       ThrowingDecls.insert(D);
     }
   };