"Too many math alphabets in version bold" error

Question

I am having a hard time with the "Too many math alphabets in version bold" error. Here is the minimal working example on the latest version of Texlive that comes with Ubuntu 18.04:

\documentclass[11pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\begin{document}
$\widetilde{\boldsymbol{\mathsf{x}}}_n$
\begin{equation}
\widetilde{\boldsymbol{\mathsf{x}}}_n
\end{equation}
\end{document}

I have been searching around but could not find the reason why this causes an error. It seems like having both \widetilde{\boldsymbol{\mathsf{x}}}_n in in-line and displayed equation causes this error, but I have absolutely no clues why and how to fix this :-(.

My question: Is there a way to fix this?

Any help/hint is highly appreciated.

PS 1: This error appears to be different than the one in "Too many math alphabets" error so I believe it is not a duplicate.

PS 2: As David Carlisle kindly pointed out in the comment, the first error in the log file is

! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup

Answer 1

I think you just need this fix

\documentclass[11pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\makeatletter
\def\@mathmeasure#1#2#3{\setbox#1\hbox{%
  \m@th$#2#3$}}
\makeatother
\begin{document}
$\widetilde{\boldsymbol{\mathsf{x}}}_n$
\begin{equation}
\tracingall
\widetilde{\boldsymbol{\mathsf{x}}}_n
\end{equation}
\end{document}

note the current amsmath source says

%    As of 2018/12 release we don't turn off |\evermath| as this
%    optimization can be harmful.
% \changes{v2.17b}{2018/12/01}{Don't drop \cs{frozen@everymath}}

That could have been me, but I don't recall, perhaps it was Frank:-)

As a general rule only look at the first error reported. The math alphabet error is entirely spurious but to get it you need to step past 18!!!! earlier error messages:

! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> \@nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> @nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> @nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> @nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> @nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> @nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> @nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> @nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Use of \reserved@a doesn't match its definition.
\reserved@b ->\relax 
                     \ifmmode \else \non@alpherr \mathsf  \fi \use@mathgroup...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? 
! Undefined control sequence.
<argument> @nil
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
?
! LaTeX Error: Too many math alphabets used in version bold.
See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...
l.8 \widetilde{\boldsymbol{\mathsf{x}}}
                                       _n
? x