Return typing of a function is ignored when calling the map() function on empty objects.

[BaileyBox]

The return types of a function are being ignored when calling the map() function on empty objects.

Here's an example of how this functioned in 1.6 vs 1.8:

Julia 1.6:

julia> function bar(v::Vector{Int})::AbstractArray{Int}
           fill(v[1], (1 for _ in 1:v[1])...)
       end
bar (generic function with 1 method)

julia> map(bar, Vector{Int}[[1]])
1-element Vector{Vector{Int64}}:
 [1]

julia> map(bar, Vector{Int}[[2]])
1-element Vector{Matrix{Int64}}:
 [2]

julia> map(bar, Vector{Int}[])
AbstractArray{Int64, N} where N[]

Julia 1.8:

julia> function bar(v::Vector{Int})::AbstractArray{Int}
           fill(v[1], (1 for _ in 1:v[1])...)
       end
bar (generic function with 1 method)

julia> map(bar, Vector{Int}[[1]])
1-element Vector{Vector{Int64}}:
 [1]

julia> map(bar, Vector{Int}[[2]])
1-element Vector{Matrix{Int64}}:
 [2;;]

julia> map(bar, Vector{Int}[])
Any[]

This was due to the changes introduced where map functions call promote_typejoin_union and return Any instead of the given return type of the function.

This functionality was always the case for broadcast(), but now since map() is following this logic, the return types are ignored on empty objects.

[vtjnash]

xref #46331

[JeffBezanson]

I must say the logic

    elseif T isa UnionAll
        return Any

is not exactly impressive.

[vtjnash]

Xref f0046a0