`sizeof` returns erroneous results for Union arrays

[iamed2]

julia> v = Union{Int,Void}[rand(Bool) ? k : nothing for k = 1:10]
10-element Array{Union{Void, Int64},1}:
   nothing
  2
  3
   nothing
   nothing
   nothing
   nothing
  8
  9
 10

julia> sizeof(v)
1762099360

Based on multiple Julia sessions, the number appears to be some random integer multiplied by the length of the array. A fresh Julia session is more likely to result in 0 so it seems like it's probably accessing the wrong memory somewhere.

[yuyichao]

Caused by #22441 ?

[iamed2]

Likely, I think

[quinnj]

Yeah, there's a few places I need to update sizeof in src; It's updated for Union{Void, Int8} for example, but not for the general object case.

[JeffBezanson]

This one looks like codegen.cpp:2726. That assumes an array element type stored unboxed is a DataType.

[vtjnash]

since we're defining sizeof to mean eltype_layout, we need to also handle abstract covariant types also (for example, Tuple{Union{Int8, Int16}} == Union{Tuple{Int8}, Tuple{Int16}})

[laborg]

Can this be closed? #38126

julia> sizeof(Vector{Union{Tuple{UInt8,UInt16}}}(undef,8))
32

julia> sizeof(Vector{Tuple{Union{UInt8,UInt16}}}(undef,4))
32

[vtjnash]

$ ./julia -q
julia> sizeof(Vector{Union{Tuple{UInt8},Tuple{UInt16}}}(undef,4))
8

julia> sizeof(Vector{Tuple{Union{UInt8,UInt16}}}(undef,4))
8

$ ./julia -q
julia> sizeof(Vector{Tuple{Union{UInt8,UInt16}}}(undef,4))
32

julia> sizeof(Vector{Union{Tuple{UInt8},Tuple{UInt16}}}(undef,4))
32