Why \IfPackageAtLeastTF{⟨package⟩}{⟨date⟩}{\PassOptionsToPackage{⟨option⟩}{⟨package⟩}}{} doesn't pass ⟨option⟩ to ⟨package⟩?

Question

My current version of parskip is 2021-03-14 so:

\IfPackageAtLeastTF{parskip}{2018-08-24}{\PassOptionsToPackage{indent}{parskip}}{}
\usepackage{parskip}

should pass the indent option to parskip. But it is not the case as pointed out by the following MCE where the paragraphs aren't indented (in the real use case, I need this conditional, not for a document, but for a package). Why?

\documentclass{article}
\IfPackageAtLeastTF{parskip}{2018-08-24}{\PassOptionsToPackage{indent}{parskip}}{}
\usepackage{parskip}
\usepackage{lipsum}
\begin{document}
\lipsum[1-2]
\end{document}

Answer 1

LaTeX only knows the version of a package (or class) once it is loaded. Before that the conditional used by \IfPackageAtLeastTF... returns false. Hence the fourth argument of

\IfPackageAtLeastTF{parskip}{2018-08-24}{\PassOptionsToPackage{indent}{parskip}}{}

is executed irrespective of parskip's version. In this case, you could use

\IfFileExists{parskip-2001-04-09.sty}{\PassOptionsToPackage{indent}{parskip}}{}