Why does \tl_if_in (and \clist_if_in) insert extra tokens with \mathrm

Question

I encountered a strange behaviour I can’t explain. In my code for a highlighting macro I tried to use \tl_if_in to do a basic math class test. But when \mathrm is part of the tokens to be tested some extra tokes (i.e. what follows \mathrm{…}) are inserted.

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn

\tl_const:Nn \c_pirkl_math_rel_symbols_tl { +-=/ }

\NewDocumentCommand{ \highlight }{ m }{
   \tl_if_in:NnT \c_pirkl_math_rel_symbols_tl { #1 } {
      % ...
   }
   #1
}

\ExplSyntaxOff

\begin{document}
Inserts what comes after \verb|\mathrm{...}|
\[ \highlight{x} + \highlight{\mathrm{d}x} \]

works with additional group
\[ \highlight{x} + \highlight{{\mathrm{d}x}} \]
\end{document}

It is the same with \clist_is_in, except that there is also an additional comma in the output.

---

note: Actually my question is kind of xy problem, I came to this while working on a solution for capturing the math class: http://tex.stackexchange.com/q/369903/4918.

---

update: with \tl_to_string added the test is allay true …

\NewDocumentCommand{ \highlight }{ m }{
   \tl_if_in:NnT \c_pirkl_math_rel_symbols_tl { \tl_to_string:n { #1 } } {
      T
   }
   #1
}

update 2: with most TL stringified I can’t make it work either …

\tl_const:Nn \c_pirkl_math_rel_symbols_tl { \tl_to_string:n { +-=/ } }

\cs_generate_variant:Nn \tl_if_in:NnT { NfT }

\NewDocumentCommand{ \highlight }{ m }{
   \tl_if_in:NfT \c_pirkl_math_rel_symbols_tl { \tl_to_string:n { #1 } } {
      T
   }
   #1
}

Answer 1

It's documented: the <token list> to search for cannot contain { or }.

Stringify both TL (constant and test) should do the trick, but you need two additional cs variants for this solution:

\cs_generate_variant:Nn \tl_if_in:NnT { NfT }

\tl_const:Nx \c_pirkl_math_rel_symbols_tl { \tl_to_str:n { +-=/ } }

\NewDocumentCommand{ \highlight }{ m }{
   \tl_if_in:NfT \c_pirkl_math_rel_symbols_tl { \tl_to_str:n { #1 } } {
      T
   }
   #1
}