Array{Missing} breaks matrix multiplication in some cases

[henriquebecker91]

Based on this discourse thread.

In Julia 1.5.3:

julia> omega = randn(10,10)
10×10 Array{Float64,2}:
...
julia> a = Array{Missing}(missing, 10, 1)
10×1 Array{Missing,2}:
...
julia> result = a' * omega * a
ERROR: InexactError: trunc(Int64, Inf)
Stacktrace:
...
julia> result =  (omega * a)
10×1 Array{Missing,2}:
...
julia> result =  (a' * omega)
1×10 Array{Missing,2}:
...
julia> a = Array{Union{Missing, Float64}}(missing, 10, 1)
10×1 Array{Union{Missing, Float64},2}:
...
julia> result = a' * omega * a
1×1 Array{Union{Missing, Float64},2}:
 missing

Not sure of a bug or using Array{Missing, 2} is something to be avoided, but it seems inconsistent that it works on some cases and not in others. If Array{Union{Missing, Float64}} is used it always seem to work.

[simeonschaub]

No, this should definitely work. Just looks like our generic matmul currently assumes one of the element types is not of size zero, which fails if you multiply two arrays of missing:

julia/stdlib/LinearAlgebra/src/matmul.jl

Line 826 in 770d0d5

tile_size = floor(Int, sqrt(tilebufsize / max(sizeof(R), sizeof(S), sizeof(T))))

Probably easy enough to fix though.