Handling of <:Union{}

[bkamins]

I think this should not be allowed:

julia> f(::Type{Union{}}) = "a"
f (generic function with 1 method)

julia> f(::Type{<:Union{}}) = "b"
f (generic function with 2 methods)

julia> methods(f)
# 2 methods for generic function "f":
[1] f(::Type{var"#s1"} where var"#s1"<:Union{}) in Main at REPL[2]:1
[2] f(::Type{Union{}}) in Main at REPL[1]:1

julia> f(Union{})
"b"

and the compiler should identify that these two signatures are identical as Union{} is bottom type.

[martinholters]

Indeed, given all of these:

julia> Type{Integer} == Type{T} where {Integer<:T<:Integer}
true

julia> Type{Int} == Type{T} where {Int<:T<:Int}
true

julia> Type{Any} == Type{T} where {Any<:T<:Any}
true

julia> Ref{Union{}} == Ref{T} where {Union{}<:T<:Union{}}
true

It's hard to justify that:

julia> Type{Union{}} == Type{T} where {Union{}<:T<:Union{}}
false