Strange expand on tcolorbox

Question

I don't understand why the following code fails with the error

ERROR: File ended while scanning use of \thmt@collect@body.

Indeed when I define

before upper={\begin{thm}[my title]}

it works, but when I use

before upper={\begin{restatable}[my title]{thm}{myrestatecommand}}

it fails, while the code should be equivalent. Any idea why?

MWE:

\documentclass{article}

\usepackage{mathtools}
\usepackage{amssymb, amsthm, amsmath, thm-restate}
\usepackage{thmtools} %%
\usepackage{hyperref}

\newtheorem{thm}{Theorem}[section]
\providecommand*\thmautorefname{Theorem}
\newtheorem{corollary}[thm]{Corollary}
\providecommand*\corollaryautorefname{Corollary}
\newtheorem{lemma}[thm]{Lemma}
\providecommand*\lemmaautorefname{Lemma}

\usepackage{lipsum}
\usepackage[most]{tcolorbox}

% Works:
\NewTColorBox[auto counter, number within=section]{theorem}{}{ %
    blank,
    breakable,
    before upper={\begin{thm}[my title]},
      after upper={\end{thm}},
}

% Fails:
\NewTColorBox[auto counter, number within=section]{theoremRestate}{}{ %
    blank,
    breakable,
    before upper={\begin{restatable}[my title]{thm}{myrestatecommand}},
      after upper={\end{restatable}},
}

\begin{document}

%% Works:
\begin{restatable}[my title]{thm}{myrestatecommanda}
  This theorem works
\end{restatable}

%% Works:
%% Does not work:
\begin{theorem}
  This theorem does not work
\end{theorem}

%% Does not work:
\begin{theoremRestate}
  This theorem does not work
\end{theoremRestate}

\end{document}

Answer 1

If your goal is just to restate tcolorbox theorems, this is possible with keytheorems (disclaimer, I wrote the package).

\documentclass{article}

\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{keytheorems}
\usepackage{kantlipsum}
\usepackage[most]{tcolorbox}
\usepackage{hyperref}

\newkeytheorem{thm}[
  name=Theorem,
  parent=section,
  tcolorbox-no-titlebar={
    breakable,
    %<other options>,
    }
  ]
\newkeytheorem{lemma}[
  sibling=thm,
  tcolorbox={
    breakable,
    %<other options>,
    }
  ]

\begin{document}

\begin{thm}[store=my:thm]
\kant[1-2]
\end{thm}

\begin{lemma}
\kant[3-4]
\end{lemma}

\getkeytheorem{my:thm}

\end{document}

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