Suboptimal performance of captured small unions

[bramtayl]

function test(ys)
    x = if rand(Bool)
        1
    else
        missing
    end
    map(y -> x + y, ys)
end
@code_warntype test((1, 2))

[Keno]

This corresponds to a TODO in the apply_type tfunc: https://github.com/JuliaLang/julia/blob/master/base/compiler/tfuncs.jl#L998. However, I think there is a larger question of whether putting the union on the outside of the closure is really the right thing to do. Perhaps there is a lowering that would allow small unions as the field type for closures.

[bramtayl]

I used this to get around it:

partial_map(f, fixed, variables::Tuple{}) = ()
partial_map(f, fixed, variables::Tuple) =
    f(fixed, variables[1]), partial_map(f, fixed, tail(variables))...
partial_map(f, fixed, ::Tuple{}, ::Tuple{}) = ()
partial_map(f, fixed, variables1::Tuple, variables2::Tuple) =
    f(fixed, variables1[1], variables2[1]),
    partial_map(f, fixed, tail(variables1), tail(variables2))...

[bramtayl]

I watched the JuliaCon talk where @StefanKarpinski mentioned prioritizing making Julia faster for tabular data and I think solving this would be an important step