Markdown reader: fix blindspot with superscript in links.

Previously `[^super^](#ref)` wasn't parsed as a link, due to
code that was meant to prevent footnote markers from being
recognized as reference links.  This commit tightens up that
code to avoid this bad effect.

We have also added a new restriction on footnote labels: they
cannot contain the characters `^`, `[`, or `]`. Though this is
technically a breaking change, we suspect that the impact will
be minimal, as it's very unlikely people would be using these
characters in their note labels.

Closes #8981.

MANUAL.txt

@@ -5509,9 +5509,9 @@ Pandoc's Markdown allows footnotes, using the following syntax:
     isn't indented.
 
 The identifiers in footnote references may not contain spaces, tabs,
-or newlines.  These identifiers are used only to correlate the
-footnote reference with the note itself; in the output, footnotes
-will be numbered sequentially.
+newlines, or the characters `^`, `[`, or `]`. These identifiers
+are used only to correlate the footnote reference with the note
+itself; in the output, footnotes will be numbered sequentially.
 
 The footnotes themselves need not be placed at the end of the
 document.  They may appear anywhere except inside other block elements

src/Text/Pandoc/Readers/Markdown.hs

@@ -142,12 +142,6 @@ isHruleChar _   = False
 setextHChars :: [Char]
 setextHChars = "=-"
 
-isBlank :: Char -> Bool
-isBlank ' '  = True
-isBlank '\t' = True
-isBlank '\n' = True
-isBlank _    = False
-
 --
 -- auxiliary functions
 --
@@ -416,7 +410,9 @@ abbrevKey = do
     return $ return mempty
 
 noteMarker :: PandocMonad m => MarkdownParser m Text
-noteMarker = string "[^" >> many1TillChar (satisfy $ not . isBlank) (char ']')
+noteMarker = string "[^" >>
+  many1TillChar (satisfy (`notElem` ['\r','\n','\t',' ','^','[',']']))
+                (char ']')
 
 rawLine :: PandocMonad m => MarkdownParser m Text
 rawLine = try $ do
@@ -1815,7 +1811,7 @@ endline = try $ do
 -- a reference label for a link
 reference :: PandocMonad m => MarkdownParser m (F Inlines, Text)
 reference = do
-  guardDisabled Ext_footnotes <|> notFollowedBy' (string "[^")
+  guardDisabled Ext_footnotes <|> notFollowedBy' noteMarker
   withRaw $ trimInlinesF <$> inlinesInBalancedBrackets
 
 parenthesizedChars :: PandocMonad m => MarkdownParser m Text

test/command/8981.md

@@ -0,0 +1,6 @@
+```
+% pandoc --wrap=none
+consectetur [^\[link\]^](#ref "Title")
+^D
+<p>consectetur <a href="#ref" title="Title"><sup>[link]</sup></a></p>
+```