How to add `parsep` to `itemsep` and set `parsep` to 0pt, when declaring my list?

Question

By reading \topsep, \itemsep, \partopsep and \parsep - what does each of them mean (and what about the bottom)?, I learned how spacing works, therefore I am trying to do some spacing settings/adjusting.

Initially I tried this:

\documentclass[12pt]{article}

\usepackage[T1]{fontenc}
\usepackage[brazil]{babel}
\usepackage[a4paper, margin=2cm]{geometry}

\usepackage[utf8]{inputenc}
\usepackage[shortlabels]{enumitem}

\begin{document}

    \begin{enumerate}[itemsep=\parsep+\itemsep,parsep=0pt] % <--- Here

        \item https://github.com/jbeder/yaml-cpp

            Some text.

        \item https://github.com/trishume/syntect

    \end{enumerate}

\end{document}

But the the compiler seems not liked:

test.tex:12: Missing number, treated as zero. [...rate}[itemsep=\parsep+\itemsep,parsep=0pt]]
test.tex:12: Illegal unit of measure (pt inserted). [...rate}[itemsep=\parsep+\itemsep,parsep=0pt]]

---

Update

I would like to:

1. Sum the parsep and itemsep values
2. Attribute them to my list option itemsep
3. And set parsep to zero.

This is a picture about it from [1]:

Answer 1

It seems a slightly strange setting for the list but the arithmetic can be performed using an e-tex dimension expression.

\dimexpr\parsep+\itemsep\relax

Answer 2

A more LaTeXy way to do this is to load the calc package, and then you don't need the \dimexpr and \relax in David's answer.

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[brazil]{babel}
\usepackage[a4paper, margin=2cm]{geometry}

\usepackage[shortlabels]{enumitem}
\usepackage{calc}

\begin{document}

\begin{enumerate}[itemsep=\parsep+\itemsep,parsep=0pt] 

    \item https://github.com/jbeder/yaml-cpp

        Some text.

    \item https://github.com/trishume/syntect

\end{document}