From 1a2ac4120a1e3d34a02cdd6e5e45cfd317125076 Mon Sep 17 00:00:00 2001
From: David Carlisle <d.p.carlisle@gmail.com>
Date: Mon, 15 Feb 2021 12:27:21 +0000
Subject: [PATCH] avoid infinite loop on math alphabet error #501 (#503)

---
 base/changes.txt               |  5 +++++
 base/ltfssdcl.dtx              |  8 ++++++--
 base/testfiles/github-0501.lvt | 14 ++++++++++++++
 base/testfiles/github-0501.tlg |  9 +++++++++
 4 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 base/testfiles/github-0501.lvt
 create mode 100644 base/testfiles/github-0501.tlg

diff --git a/base/changes.txt b/base/changes.txt
index 1424800a2..67f9c839d 100644
--- a/base/changes.txt
+++ b/base/changes.txt
@@ -6,6 +6,11 @@ completeness or accuracy and it contains some references to files that
 are not part of the distribution.
 ================================================================================
 
+2021-02-15  David Carlisle  <David.Carlisle@latex-project.org>
+
+	* ltfssdcl.dtx: two additional \expandafter to avoid infinite
+	loop on not in math error. (gh/501)
+
 2021-02-11  Frank Mittelbach  <Frank.Mittelbach@latex-project.org>
 
 	* classes.dtx (subsubsection{Itemize}):
diff --git a/base/ltfssdcl.dtx b/base/ltfssdcl.dtx
index dd4e14fa8..9710d5438 100644
--- a/base/ltfssdcl.dtx
+++ b/base/ltfssdcl.dtx
@@ -36,7 +36,7 @@
 %
 %
 \ProvidesFile{ltfssdcl.dtx}
-            [2021/01/15 v3.0v LaTeX Kernel (NFSS Declarative Interface)]
+            [2021/02/15 v3.0w LaTeX Kernel (NFSS Declarative Interface)]
 % \iffalse
 \documentclass{ltxdoc}
 \begin{document}
@@ -490,7 +490,11 @@
                    in version \math@version}%
         \@eha
   \fi
- \else \expandafter\non@alpherr\fi
+%    \end{macrocode}
+% extra \cs{expandafter} to remove the \cs{expandafter} added below
+% \changes{v3.0w}{2021/02/115}{fix for (gh/501)}
+%    \begin{macrocode}
+ \else \expandafter\expandafter\expandafter\non@alpherr\fi
 %    \end{macrocode}
 %    If the legacy interface is used, e.g., \verb=$\sf -1$= the math
 %    alphabet \verb=#1= does not take an argument so we better do not
diff --git a/base/testfiles/github-0501.lvt b/base/testfiles/github-0501.lvt
new file mode 100644
index 000000000..9ad2a9b0c
--- /dev/null
+++ b/base/testfiles/github-0501.lvt
@@ -0,0 +1,14 @@
+\documentclass{article}
+
+\input{test2e}
+
+\begin{document}
+
+\START
+
+% should give a single error on \mathit
+% not loop errors on \expandafter
+
+\mathit{x}
+
+\END
\ No newline at end of file
diff --git a/base/testfiles/github-0501.tlg b/base/testfiles/github-0501.tlg
new file mode 100644
index 000000000..ee87539b1
--- /dev/null
+++ b/base/testfiles/github-0501.tlg
@@ -0,0 +1,9 @@
+This is a generated file for the LaTeX2e validation system.
+Don't change this file in any respect.
+! LaTeX Error: \mathit allowed only in math mode.
+See the LaTeX manual or LaTeX Companion for explanation.
+Type  H <return>  for immediate help.
+ ...                                              
+l. ...\mathit{x}
+You're in trouble here.  Try typing  <return>  to proceed.
+If that doesn't work, type  X <return>  to quit.