type instability when swapping two integers and feeding a sum

[lmiq]

Probably a duplicate of some known issue, but anyway:

This is the MWE:

function unstable(x,k,l)
    if k > l; k, l = l, k; end
    return sum((x[k]+x[l])^2 for _ in eachindex(x)) 
end

function stable(x,k,l)
    k, l = if k > l; (l, k); else (k, l); end
    return sum((x[k]+x[l])^2 for _ in eachindex(x)) 
end

julia> @b unstable(1:100, 2, 10)
4.478 μs (299 allocs: 7.797 KiB)

julia> @b stable(1:100, 2, 10)
1.335 ns

(from: https://discourse.julialang.org/t/help-with-optimising-a-simple-function/131459/6)

[KristofferC]

Is the the classic boxing of captured closure variables?