Integrate[Sqrt[x^3 + x], x] doesn't work

[halirutan]

The following antiderivative cannot be found by Rubi. However, it doesn't say so but rather brings many messages:

$LoadElementaryFunctionRules = True;
<< Rubi`

Integrate[Sqrt[x^3 + x], x]

This results in the following. Why is that?

[AlbertRich]

The Rubi 4.15.2 available on the UWO website gets it in 6 steps. What version are you running? Try single stepping to see where it goes haywire.

[halirutan]

I'm using version 4.15.2.1. The error comes from evaluating this Int along the way

Int[1/Sqrt[1 + x^4] - x^2/Sqrt[1 + x^4], x]

For expression, the very early 13th DownValue matches

HoldPattern[Int[u_, x_Symbol]] :> Simp[IntSum[u, x], x] /; SumQ[Rubi`Private`u]

Therefore, this call goes haywire on my machine

Rubi`Private`IntSum[1/Sqrt[1 + x^4] - x^2/Sqrt[1 + x^4], x]

[halirutan]

Fixed in 4.16.0