A Union of Tuple types isa Tuple type ?!

General Usage
1

So, what’s the rationale for the following behaviour, which caught me by surprise:

julia> Union{Tuple{Int}, Tuple{Char}} <: Tuple
true
2

Any instance of Union{Tuple{Int}, Tuple{Char}} is also an instance of Tuple. A simpler example is

julia> Union{Int32,Int64} <: Integer
true

But the title of your question says isa, which isn’t true:

julia> Union{Tuple{Int}, Tuple{Char}} isa Tuple
false
3

if everything in a Union <: T, then this Union <: T, after all, a Union is nothing but a “union” of types; Union should not be thought of as a type like Tuple or Set or Vector, similar to how DataType shouldn’t be thought as “ordinary” types like Float64 or Integer (thus the special behavior)

4

Yes, of course. Many thanks.