IRGen: Async throwing functions with empty error types pass the error indirectly

rdar://135304464

Author: aschwaighofer

lib/IRGen/GenCall.cpp

@@ -3030,7 +3030,10 @@ class AsyncCallEmission final : public CallEmission {
             IGF.IGM.getTypeInfo(silErrorTy).nativeReturnValueSchema(IGF.IGM);
 
         if (nativeSchema.requiresIndirect() ||
-            errorSchema.shouldReturnTypedErrorIndirectly()) {
+            errorSchema.shouldReturnTypedErrorIndirectly() ||
+            errorSchema.empty()) { // direct empty typed errors are passed
+                                   // indirectly for compatibility with generic
+                                   // functions.
           // Return the error indirectly.
           auto buf = IGF.getCalleeTypedErrorResultSlot(silErrorTy);
           Args[--LastArgWritten] = buf.getAddress();

test/IRGen/typed_throws.sil

@@ -458,3 +458,34 @@ bb6:
   %t = tuple()
   return %t : $()
 }
+
+sil @callee : $@convention(thin) @async (Int64) -> (@out (), @error Never) {
+bb0(%0 : $*(), %1: $Int64):
+  %17 = tuple ()
+  return %17 : $()
+}
+
+// CHECK: define{{.*}} swifttailcc void @callee(ptr {{.*}} %0, ptr swiftasync %1, i64 %2, ptr %3)
+
+// CHECK: define{{.*}} swifttailcc void @caller(ptr swiftasync %0) {{.*}} {
+// CHECK-NOT: define
+// CHECK: [[CTXT:%.*]] = call swiftcc ptr @swift_task_alloc
+// CHECK-NOT: define
+// CHECK: call {{.*}} @llvm.coro.suspend.async.sl_p0p0s(i32 256, ptr {{.*}}, ptr @__swift_async_resume_project_context, ptr @caller.0, ptr @callee, ptr undef, ptr [[CTXT]], i64 66, ptr %swifterror)
+
+sil @caller : $@convention(thin) @async () -> () {
+bb0:
+  %5 = integer_literal $Builtin.Int64, 66
+  %6 = struct $Int64 (%5 : $Builtin.Int64)
+  %9 = alloc_stack $()
+  %10 = function_ref @callee : $@convention(thin) @async (Int64) -> (@out (), @error Never)
+  try_apply %10(%9, %6) : $@convention(thin) @async (Int64) -> (@out (), @error Never), normal bb1, error bb2
+
+bb1(%12 : $()):
+  dealloc_stack %9 : $*()
+  %17 = tuple ()
+  return %17 : $()
+
+bb2(%19 : $Never):
+  unreachable
+}