Don't treat a citation as a reference link label.

Closes #1763.

README

@@ -2400,7 +2400,9 @@ The link consists of link text in square brackets, followed by a label in
 square brackets. (There can be space between the two.) The link definition
 consists of the bracketed label, followed by a colon and a space, followed by
 the URL, and optionally (after a space) a link title either in quotes or in
-parentheses.
+parentheses.  The label must not be parseable as a citation (assuming
+the `citations` extension is enabled):  citations take precedence over
+link labels.
 
 Here are some examples:
 

src/Text/Pandoc/Readers/Markdown.hs

@@ -1675,9 +1675,10 @@ referenceLink :: (String -> String -> Inlines -> Inlines)
               -> (F Inlines, String) -> MarkdownParser (F Inlines)
 referenceLink constructor (lab, raw) = do
   sp <- (True <$ lookAhead (char ' ')) <|> return False
-  (ref,raw') <- try
+  (ref,raw') <- option (mempty, "") $
-           (skipSpaces >> optional (newline >> skipSpaces) >> reference)
+      lookAhead (try (spnl >> normalCite >> return (mempty, "")))
-           <|> return (mempty, "")
+      <|>
+      try (spnl >> reference)
   let labIsRef = raw' == "" || raw' == "[]"
   let key = toKey $ if labIsRef then raw else raw'
   parsedRaw <- parseFromString (mconcat <$> many inline) raw'

tests/markdown-reader-more.native

@@ -152,6 +152,8 @@
 ,Para [Link [Str "linky"] ("hi_(there_(nested))","")]
 ,Header 2 ("reference-link-fallbacks",[],[]) [Str "Reference",Space,Str "link",Space,Str "fallbacks"]
 ,Para [Str "[",Emph [Str "not",Space,Str "a",Space,Str "link"],Str "]",Space,Str "[",Emph [Str "nope"],Str "]\8230"]
+,Header 2 ("reference-link-followed-by-a-citation",[],[]) [Str "Reference",Space,Str "link",Space,Str "followed",Space,Str "by",Space,Str "a",Space,Str "citation"]
+,Para [Str "MapReduce",Space,Str "is",Space,Str "a",Space,Str "paradigm",Space,Str "popularized",Space,Str "by",Space,Link [Str "Google"] ("http://google.com",""),Space,Cite [Citation {citationId = "mapreduce", citationPrefix = [], citationSuffix = [], citationMode = NormalCitation, citationNoteNum = 0, citationHash = 0}] [Str "[@mapreduce]"],Space,Str "as",Space,Str "its",Space,Str "most",Space,Str "vocal",Space,Str "proponent."]
 ,Header 2 ("empty-reference-links",[],[]) [Str "Empty",Space,Str "reference",Space,Str "links"]
 ,Para [Str "bar"]
 ,Para [Link [Str "foo2"] ("","")]]

tests/markdown-reader-more.txt

@@ -262,6 +262,13 @@ Empty cells
 
 [*not a link*] [*nope*]...
 
+## Reference link followed by a citation
+
+MapReduce is a paradigm popularized by [Google] [@mapreduce] as its
+most vocal proponent.
+
+[Google]: http://google.com
+
 ## Empty reference links
 
 [foo2]: