@bew those restrictions are not missing. Adding them would make the API of hash a bit more pedantic specially when dealing with unions. If the key comes from a union you will need to ensure the type match before calling. And also when modules and type hierarchy are involved and the inferred type does not match what you expect. I recall going back and forth in this manner in the whiteboard.

It's true that returning always false vs not compiling seems less safety or bug prone. But in containers API I found that sometime is more a grayscale than black or white criteria.