LaTeX reader: more robust parsing of bracketed options.

Improves on 9a40976.  Closes #6873.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -559,9 +559,14 @@ tok = try $ spaces >> grouped inline <|> inlineCommand' <|> singleChar'
           return $ str t
 
 opt :: PandocMonad m => LP m Inlines
-opt = bracketed inline
-      <|>
-      (str . T.dropWhile (=='[') . T.dropWhileEnd (==']') <$> rawopt)
+opt = do
+  toks <- try (sp *> bracketedToks <* sp)
+  -- now parse the toks as inlines
+  st <- getState
+  parsed <- runParserT (mconcat <$> many inline) st "bracketed option" toks
+  case parsed of
+    Right result -> return result
+    Left e       -> throwError $ PandocParsecError (untokenize toks) e
 
 paropt :: PandocMonad m => LP m Inlines
 paropt = parenWrapped inline

test/command/6873.md

@@ -0,0 +1,7 @@
+```
+% pandoc -f latex -t native --citeproc
+\cite[„Etwas […{]} auslassen“]{key}
+^D
+[WARNING] Citeproc: citation key not found
+[Para [Cite [Citation {citationId = "key", citationPrefix = [], citationSuffix = [Str "\8222Etwas",Space,Str "[\8230",Span ("",[],[]) [Str "]"],Space,Str "auslassen\8220"], citationMode = NormalCitation, citationNoteNum = 0, citationHash = 0}] [Str "(",Strong [Str "key?"],Str "\8222Etwas",Space,Str "[\8230",Span ("",[],[]) [Str "]"],Space,Str "auslassen\8220)"]]]
+```