Why don't `Type` selectors act as supertypes in `Tuple` types?

General Usage
, ,
1 
julia> 1 isa Int
true

julia> (1,) isa Tuple{Int}
true

julia> Int isa Type{Int}
true

julia> (Int,) isa Tuple{Type{Int}} # ???
false

And how does dispatch work if this supertyping doesn’t work for method signatures as tuple types?

2

typeof((Int,)) is not Tuple{Type{Int}}, but Tuple{DataType}, and accordingly this does work:

julia> (Int,) isa Tuple{DataType}
true

The point as I understand it is related to:

julia> isconcretetype(DataType)
true

julia> isconcretetype(Type{Int})
false

Counterintuitive? I’d say yes, but correctly documented:

For each type T, Type{T} is an abstract parametric type whose only instance is the object T.

(emphasis added).

This means that once you wrap a DataType (Int, Float64…) inside a Tuple, it is no longer possible to distinguish it from any other DataType by looking into the type of that tuple, just as you cannot distinguish (1,) from (2,) only by the tuple’s type - you have to access directly the members of the tuple to do it.