Generic type parametrized by supertype

[lmiq]

The problem there is that Int64 is concrete and cannot be subtyped. With Integer it should work.

Why is it impossible to subtype a struct?

Multiple inheritance is a totally different story and could be allowed. I’ve written about why we don’t allow subtyping of concrete types before if someone wants to dig up one of my earlier posts about it. Edit: here’s a link to an old google groups conversation about it https://groups.google.com/d/msg/julia-dev/eA4VkFAD-yQ/LNUP_OT0zy0J In particular, this paragraph from the end of my first post on the thread sums up my position fairly well still: While there are a number of practical reaso…