Biber 2.4, microtype, and special characters

I'd say this is a bug in biblatex:

In biblatex_.sty (which is the non-legacy version that is used with biber), we find right at the beginning:

% Tests for emptiness, regardless whether the arg is macro or literal
% Expand everything and test as string
\def\ifempty#1{%
  \edef\z{#1}%
  \expandafter\ifblank\expandafter{\z}}

Now, \edef'ing user input is something that should never be done, period.

The OP's minimal example works for me if I replace \edef with \protected@edef (which is latex's "user-safe" version of \edef), but I cannot foresee which other consequences this might have.

This was, due to a biblatex change. I was trying to insulate users as much as possible from the namepart changes but this isn't really feasible because of this sort of consequence. As mentioned, the problem doesn't exist if you use fontenc which I assume you should be doing anyway. However, I have changed the problematic test in biblatex 3.4 on DEV and the problem no longer occurs.

I get no error if I just add \usepackage[T1]{fontenc}. It's not really clear where microtype chokes when the encoding is OT1.

biblatex 2016/03/03 v3.3 and biber version: 2.4

\RequirePackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{renyi1970,
  author = {R\'enyi, Alfr\'ed},
  location = {San Francisco},
  publisher = {Holden-Day},
  title = {Foundations of Probability},
  year = {1970},
}
\end{filecontents}

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[backend=biber]{biblatex}
\usepackage{microtype}

\addbibresource{\jobname.bib}

\begin{document}

\cite{renyi1970}

\printbibliography

\end{document}