LaTeX reader: fix bug parsing macro arguments.

If `\cL` is defined as `\mathcal{L}`, and `\til` as `\tilde{#1}`,
then `\til\cL` should expand to `\tilde{\mathcal{L}}`, but pandoc
was expanding it to `\tilde\mathcal{L}`.  This is fixed by
parsing the arguments in "verbatim mode" when the macro expands
arguments at the point of use.

Closes #6796.

src/Text/Pandoc/Readers/LaTeX/Parsing.hs

@@ -484,7 +484,11 @@ doMacros' n inp =
            Nothing -> mzero
            Just (Macro expansionPoint argspecs optarg newtoks) -> do
              let getargs' = do
-                   args <- case optarg of
+                   args <-
+                     (case expansionPoint of
+                        ExpandWhenUsed    -> withVerbatimMode
+                        ExpandWhenDefined -> id)
+                     $ case optarg of
                              Nothing -> getargs M.empty argspecs
                              Just o  -> do
                                 x <- option o bracketedToks

test/command/6796.md

@@ -0,0 +1,16 @@
+```
+% pandoc -f latex -t markdown
+\newcommand{\cL}{\mathcal{L}}
+\newcommand{\til}[1]{\tilde{#1}}
+
+$$\til\cL$$
+
+\newcommand{\mc}[1]{\mathcal{#1}}
+\newcommand{\dL}{\mc{L}}
+
+$$\til\dL$$
+^D
+$$\tilde{\mathcal{L}}$$
+
+$$\tilde{\mathcal{L}}$$
+```