I see. I can not give more insightful answer here then. From personal experience, I've noticed with 0.16 with the std.Io async stuff that you cannot do:

   io.concurrent(foo, .{});

where foo's return type is `error{foobar}!noreturn`, because the compiler crashes when it tries to use that type as a std.Io.Future(T)'s struct field. Might be related or not.