I would have thought that if I declare \let\stdeqref\eqref, then the \stdeqref macro and the \eqref macro would be interchangeable. But here is an example where it fails! Moreover, this failure is (somehow) caused by the microtype package?! What is going on, and how to fix it?

\documentclass{article}

\usepackage{microtype}

\usepackage{amsmath}

\let\stdeqref\eqref

\begin{document}

\begin{equation}\label{a}
A
\end{equation}

This is good spacing: \eqref{a}\allowbreak\eqref{a}

This is bad spacing: \stdeqref{a}\allowbreak\stdeqref{a}

But the bad spacing becomes good if we remove microtype!

\end{document}

The reason I'm asking is that I would like to redefine the \eqref macro in a way which uses the usual \eqref macro as a subcomponent, via the usual strategy I've seen time and time again on this site:

\let\stdeqref\eqref
\renewcommand\eqref[1]{Something fancy containing \stdeqref{#1} etc.}

but that doesn't work anymore if \let isn't behaving as expected . . .

I would have thought that if I declare \let\stdeqref\eqref, then the \stdeqref macro and the \eqref macro would be interchangeable. But here is an example where it fails! Moreover, this failure is (somehow) caused by the microtype package?! What is going on, and how to fix it?

\documentclass{article}

\usepackage{microtype}

\usepackage{amsmath}

\let\stdeqref\eqref

\begin{document}

\begin{equation}\label{a}
A
\end{equation}

This is good spacing: \eqref{a}\allowbreak\eqref{a}

This is bad spacing: \stdeqref{a}\allowbreak\stdeqref{a}

But the bad spacing becomes good if we remove microtype!

\end{document}

The reason I'm asking is that I would like to redefine the \eqref macro in a way which uses the usual \eqref macro as a subcomponent, via the usual strategy I've seen time and time again on this site:

\let\stdeqref\eqref
\renewcommand\eqref[1]{Something fancy containing \stdeqref{#1} etc.}

but that doesn't work anymore if \let isn't behaving as expected . . .