\documentclass{article}
\usepackage{thm-restate} % from CTAN package thmtools
\usepackage{breqn}
\usepackage{cleveref}
\begin{document}
\begin{restatable}{theorem}{foo}
Test
\end{restatable}
\end{document}
See https://tex.stackexchange.com/questions/527810 for details and a detailed analysis by Phelype Oleinik. He suggests in the comments to fix this in breqn because thmtools is not maintained.
The following does not compile:
See https://tex.stackexchange.com/questions/527810 for details and a detailed analysis by Phelype Oleinik. He suggests in the comments to fix this in
breqn
because thmtools is not maintained.