This is exactly right. If the spec defined an API with simple semantics half these problems wouldn't exist. As it is `then` is overloaded to perform numerous di

This is exactly right. If the spec defined an API with simple semantics half these problems wouldn't exist. As it is `then` is overloaded to perform numerous distinct operations. This means it can't report any useful errors, because any value passed to `then` is valid. There was even discussion about this when the spec was being written, but it was decided the current semantics are more in keeping with Javascript. Unfortunately the spec authors were correct.