Make Union{} <: T always return false (except when T is Union{})

[davidanthoff]

I have to admit that I don't understand the formal properties of the type system well enough to tell whether this could cause trouble, and also not clear whether this could be done in a 1.x release at all. With those caveats:

The current behavior, that any subtype check with Union{} on the left hand side will return true (no matter what) was surprising to me and makes it kind of cumbersome to write code that really does what at least I want it to do when I'm checking for a subtype relationship. I've always assumed that if I write say T <: AbstractString, that I really only can get a true if T is some sort of string, but it turns out that if I want that behavior I need to write T <: AbstractString && T !== Union{}.

This came up in queryverse/TableTraits.jl#21.

[simeonschaub]

This is the defining property of the bottom type, so changing this would pretty much break all the internals related to the type system. What has come up before though is being able to dispatch on Type{T} where {T <: S}, but excluding T == Union{}, which is often useful when hunting down invalidations.

[davidanthoff]

This is the defining property of the bottom type, so changing this would pretty much break all the internals related to the type system.

See, I knew I was missing at least some minor aspect of this ;)

I think what is still true, though, is that in pretty much every instance where I have used the <: function in my code ever, I actually didn't really want to test for the subtype relationship as defined in Julia (and that I now understand), but instead had hoped to get something that is more narrow and excludes the bottom type, like what apparently has been discussed for dispatch as well.

Don't really know whether I have a real suggestion here, other than a) it would be nice if there was a name for the relationship I'm interested in and b) if there was some function or operator in Julia that I could use instead of <: out of the box. I guess for now I can just create a need_a_name_for_this_function(T1,T2) = T1===T2 || (T1 <: T2 && T1 !== Union{}).

[c42f]

Possible duplicate of #33780 ?