From a80ddb81b30cb8b546b0ec92b3209daad4f238e5 Mon Sep 17 00:00:00 2001
From: Erik Demaine <edemaine@mit.edu>
Date: Tue, 1 Nov 2022 17:53:24 -0400
Subject: [PATCH] fix: space should prevent optional argument to \\

Fix `src/functions/cr.js`'s definition of `\\` to manually look for an
optional argument via `future()` instead of `numOptionalArgs`, so that
it does *not* skip over spaces.  This matches the existing behavior of
`\\` in `src/environments/array.js` and AMSMath's behavior of `\math@cr`
via `\new@ifnextchar`.

Fixes #3745
---
 src/functions/cr.js | 6 +++---
 test/katex-spec.js  | 5 +++++
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/functions/cr.js b/src/functions/cr.js
index 3fc0acde70..bf25f26d57 100644
--- a/src/functions/cr.js
+++ b/src/functions/cr.js
@@ -13,13 +13,13 @@ defineFunction({
     names: ["\\\\"],
     props: {
         numArgs: 0,
-        numOptionalArgs: 1,
-        argTypes: ["size"],
+        numOptionalArgs: 0,
         allowedInText: true,
     },
 
     handler({parser}, args, optArgs) {
-        const size = optArgs[0];
+        const size = parser.gullet.future().text === "[" ?
+            parser.parseSizeGroup(true) : null;
         const newLine = !parser.settings.displayMode ||
             !parser.settings.useStrictBehavior(
                 "newLineInDisplayMode", "In LaTeX, \\\\ or \\newline " +
diff --git a/test/katex-spec.js b/test/katex-spec.js
index 985bf64734..a2b088b323 100644
--- a/test/katex-spec.js
+++ b/test/katex-spec.js
@@ -1309,6 +1309,11 @@ describe("A begin/end parser", function() {
         expect`\begin{matrix}a&b\cr[c]&d\end{matrix}`.toParse();
     });
 
+    it("should not treat [ after space as optional argument to \\\\", function() {
+        expect`\begin{matrix}a&b\\ [c]&d\end{matrix}`.toParse();
+        expect`a\\ [b]`.toParse();
+    });
+
     it("should eat a final newline", function() {
         const m3 = getParsed`\begin{matrix}a&b\\ c&d \\ \end{matrix}`[0];
         expect(m3.body).toHaveLength(2);