widetilde from mtpro2 not working in section

The commands widecheck, widehat and widetilde are defined in terms of mathpalette; precisely, widetilde is mathpalette@widetilde (similarly for the other two) so they should be made robust.

Not that mathpalette is fragile by itself, but it's better if the .aux file reports widetilde instead of the full expansion of mathpalette@widetilde, even if it didn't raise an error.

The method is easy

documentclass{article}

usepackage[lite]{mtpro2}
usepackage{etoolbox}

robustifywidetilde
robustifywidehat
robustifywidecheck

begin{document}

section{$widetilde{O}widehat{O}widecheck{O}$}

end{document}

History

The LaTeX kernel, could not make robust that many commands, because of memory limitation. So the protect mechanism was used. In recent years, memory started to no longer be a problem and MakeRobust was added to the kernel (instead of having to be requested with fixltx2e).

How does a LaTeX robust command work? If one does DeclareRobustCommand{foo}[<n>]{...}, LaTeX will define both foo and foo• (where • stands for a space in the command name). The former is simply a shorthand for protectfoo• and it's the latter that does the real work.

Until a few years ago, widetilde was not defined as a robust command, so an input such as

documentclass{article}
usepackage[lite]{mtpro2}
MakeRobustwidetilde

was a good way to make widetilde suitable for usage in moving arguments.

Until the LaTeX kernel made a huge step in robustifying many more commands, among which the wide math accents. Good! No need of MakeRobust any longer!

Well, almost. Unfortunately, mtpro2 still does defwidetilde#1{...} and this only redefines the “surface” command widetilde, but leaves widetilde• untouched. But now widetilde is not robust and it will fail in moving arguments. So we apply again MakeRobustwidetilde, isn't that easy?

Sorry, no: MakeRobust has no way to know how a command was defined and it checks for the existence of the “inner” command (with a trailing space in its name). If it finds it, then it thinks that the command is already robust and does nothing.

So with TeX Live 2020 or later the previously suggested workaround with MakeRobustwidetilde ceased to work.

Since robustify uses a very different protection mechanism, one can get around the problem. Not a big deal: etoolbox is a well-tested and maintained package.

Otherwise one has to remove the “inner version” before applying MakeRobust.

documentclass{article}

usepackage[lite]{mtpro2}

makeatletter
newcommand{ReallyMakeRobust}[1]{%
  count@ =escapechar escapechar =`
  expandafterletcsnameexpandafter@gobblestring#1spaceendcsnamerelax
  escapechar=count@
  MakeRobust#1%
}
makeatother

ReallyMakeRobustwidetilde
ReallyMakeRobustwidehat
ReallyMakeRobustwidecheck

begin{document}

section{$widetilde{O}widehat{O}widecheck{O}$}

end{document}