Markdown reader: support GitHub wiki's internal links (#2923) (#6458)

Canges overview:

 * Add a `Ext_markdown_github_wikilink` constructor to `Extension` [API change].
 * Add the parser `githubWikiLink` in `Text.Pandoc.Readers.Markdown`
 * Add tests.

src/Text/Pandoc/Extensions.hs

@@ -118,6 +118,7 @@ data Extension =
     | Ext_literate_haskell    -- ^ Enable literate Haskell conventions
     | Ext_markdown_attribute      -- ^ Interpret text inside HTML as markdown iff
                                   --   container has attribute 'markdown'
+    | Ext_wikilinks -- ^ Interpret a markdown wiki link
     | Ext_markdown_in_html_blocks -- ^ Interpret as markdown inside HTML blocks
     | Ext_mmd_header_identifiers -- ^ Multimarkdown style header identifiers [myid]
     | Ext_mmd_link_attributes     -- ^ MMD style reference link attributes
@@ -258,6 +259,7 @@ githubMarkdownExtensions = extensionsFromList
   , Ext_emoji
   , Ext_fenced_code_blocks
   , Ext_backtick_code_blocks
+  , Ext_wikilinks
   ]
 
 -- | Extensions to be used with multimarkdown.
@@ -444,6 +446,7 @@ getAllExtensions f = universalExtensions <> getAll f
        , Ext_tex_math_single_backslash
        , Ext_tex_math_double_backslash
        , Ext_markdown_attribute
+       , Ext_wikilinks
        , Ext_mmd_title_block
        , Ext_abbreviations
        , Ext_autolink_bare_uris

src/Text/Pandoc/Readers/Markdown.hs

@@ -21,6 +21,7 @@ module Text.Pandoc.Readers.Markdown (
 import Control.Monad
 import Control.Monad.Except (throwError)
 import Data.Char (isAlphaNum, isPunctuation, isSpace)
+import Data.Functor (($>))
 import Data.List (transpose, elemIndex, sortOn)
 import qualified Data.Map as M
 import Data.Maybe
@@ -1485,6 +1486,7 @@ inline = choice [ whitespace
                 , note
                 , cite
                 , bracketedSpan
+                , githubWikiLink
                 , link
                 , image
                 , math
@@ -1780,6 +1782,29 @@ source = do
 linkTitle :: PandocMonad m => MarkdownParser m Text
 linkTitle = quotedTitle '"' <|> quotedTitle '\''
 
+-- Github wiki style link, with optional title
+-- syntax documented under https://help.github.com/en/github/building-a-strong-community/editing-wiki-content
+githubWikiLink :: PandocMonad m => MarkdownParser m (F Inlines)
+githubWikiLink = try $ guardEnabled Ext_wikilinks >> wikilink
+  where
+    wikilink = try $ do
+      string "[["
+      firstPart <- fmap mconcat . sequence <$> wikiText
+      (char '|' *> complexWikilink firstPart)
+        <|> (string "]]" $> (B.link
+                               <$> (stringify <$> firstPart)
+                               <*> return "wikilink"
+                               <*> firstPart))
+
+    complexWikilink firstPart = do
+      url <- fmap stringify . sequence <$> wikiText
+      string "]]"
+      return $ B.link <$> url
+                      <*> return "wikilink"
+                      <*> firstPart
+
+    wikiText = many (whitespace <|> bareURL <|> str <|> endline <|> escapedChar)
+
 link :: PandocMonad m => MarkdownParser m (F Inlines)
 link = try $ do
   st <- getState

test/Tests/Readers/Markdown.hs

@@ -307,6 +307,36 @@ tests = [ testGroup "inline code"
             "[https://example.org(](url)" =?>
             para (link "url" "" (text "https://example.org("))
           ]
+        , testGroup "Github wiki links"
+          [ test markdownGH "autolink" $
+            "[[https://example.org]]" =?>
+            para (link "https://example.org" "wikilink" (text "https://example.org"))
+          , test markdownGH "link with title" $
+            "[[title|https://example.org]]" =?>
+            para (link "https://example.org" "wikilink" (text "title"))
+          , test markdownGH "bad link with title" $
+            "[[title|random string]]" =?>
+            para (link "random-string" "wikilink" (text "title"))
+          , test markdownGH "autolink not being a link" $
+            "[[Name of page]]" =?>
+            para (link "Name-of-page" "wikilink" (text "Name of page"))
+          , test markdownGH "autolink not being a link with a square bracket" $
+            "[[Name of ]page]]" =?>
+            para (link "Name-of-]page" "wikilink" (text "Name of ]page"))
+          , test markdownGH "formatting (strong and emphasis) should not be interpreted" $
+             "[[***a**b **c**d*|https://example.org]]" =?>
+             para (text "[[" <> emph (strong (str "a") <> str "b" <> space
+                   <> strong (str "c") <> str "d") <> text "|https://example.org]]")
+          , test markdownGH "inlined code should not make a link" $
+            "[[ti`|`le|https://example.org]]" =?>
+            para (text "[[ti" <> code "|" <> text "le|https://example.org]]")
+          , test markdownGH "link with title and a cut should take the middle part as link" $
+            "[[tit|le|https://example.org]]" =?>
+            para (link "le" "wikilink" (text "tit"))
+          , test markdownGH "link with inline start should be a link" $
+            "[[t`i*t_le|https://example.org]]" =?>
+            para (link "https://example.org" "wikilink" (text "t`i*t_le"))
+          ]
         , testGroup "Headers"
           [ "blank line before header" =:
             "\n# Header\n"