Type parameter currying doesn't work with `Tuple`

[MasonProtter]

Regular type parameters can be 'curried':

julia> struct Foo{T, U, V, W} end

julia> Foo
Foo

julia> ans{1}
Foo{1}

julia> ans{2}
Foo{1, 2}

julia> ans{3}
Foo{1, 2, 3}

julia> ans{4}
Foo{1, 2, 3, 4}

But not Tuple:

julia> Tuple
Tuple

julia> ans{Int}
Tuple{Int64}

julia> ans{String}
ERROR: TypeError: in Type{...} expression, expected UnionAll, got Type{Tuple{Int64}}
Stacktrace:
 [1] top-level scope
   @ REPL[37]:1

This is a kinda annoying limitation sometimes when one wants to construct types iteratively, and Tuple is the most natural type to want to do that with, but the only one where it doesn't work.

[nsajko]

This would mean that type application works when the LHS is concrete, which would be a bit strange.