Remove fenced_code_blocks and backtick_code_blocks from...

commonmark/gfm extensions.  These shouldn't really be counted
as extensions, because they can't be disabled in commonmark.

Adjust markdown writer to check for commonmark variant in addition
to extensions.

src/Text/Pandoc/Extensions.hs

@@ -339,7 +339,17 @@ getDefaultExtensions "muse"            = extensionsFromList
                                            [Ext_amuse,
                                             Ext_auto_identifiers]
 getDefaultExtensions "plain"           = plainExtensions
-getDefaultExtensions "gfm"             = githubMarkdownExtensions
+getDefaultExtensions "gfm"             = extensionsFromList
+  [ Ext_pipe_tables
+  , Ext_raw_html
+  , Ext_native_divs
+  , Ext_auto_identifiers
+  , Ext_gfm_auto_identifiers
+  , Ext_autolink_bare_uris
+  , Ext_strikeout
+  , Ext_task_lists
+  , Ext_emoji
+  ]
 getDefaultExtensions "commonmark"      = extensionsFromList
                                           [Ext_raw_html]
 getDefaultExtensions "commonmark_x"    = extensionsFromList
@@ -364,9 +374,7 @@ getDefaultExtensions "commonmark_x"    = extensionsFromList
   , Ext_raw_attribute
   , Ext_implicit_header_references
   , Ext_attributes
-  , Ext_fenced_code_blocks
   , Ext_fenced_code_attributes
-  , Ext_backtick_code_blocks
   ]
 getDefaultExtensions "org"             = extensionsFromList
                                           [Ext_citations,
@@ -489,9 +497,7 @@ getAllExtensions f = universalExtensions <> getAll f
     , Ext_raw_attribute
     , Ext_implicit_header_references
     , Ext_attributes
-    , Ext_fenced_code_blocks
     , Ext_fenced_code_attributes
-    , Ext_backtick_code_blocks
     ]
   getAll "commonmark_x"    = getAll "commonmark"
   getAll "org"             = autoIdExtensions <>

src/Text/Pandoc/Writers/Markdown.hs

@@ -561,9 +561,12 @@ blockToMarkdown' opts (CodeBlock (_,classes,_) str)
   | "haskell" `elem` classes && "literate" `elem` classes &&
     isEnabled Ext_literate_haskell opts =
   return $ prefixed "> " (literal str) <> blankline
-blockToMarkdown' opts (CodeBlock attribs str) = return $
-  case attribs == nullAttr of
-     False | isEnabled Ext_backtick_code_blocks opts ->
+blockToMarkdown' opts (CodeBlock attribs str) = do
+  variant <- asks envVariant
+  return $
+   case attribs == nullAttr of
+     False | variant == Commonmark ||
+             isEnabled Ext_backtick_code_blocks opts ->
           backticks <> attrs <> cr <> literal str <> cr <> backticks <> blankline
            | isEnabled Ext_fenced_code_blocks opts ->
           tildes <> attrs <> cr <> literal str <> cr <> tildes <> blankline
@@ -856,9 +859,12 @@ blockListToMarkdown opts blocks = do
   -- b) change Plain to Para unless it's followed by a RawBlock
   -- or has a list as its parent (#3487)
   let fixBlocks (b : CodeBlock attr x : rest)
-         | (not (isEnabled Ext_fenced_code_blocks opts) || attr == nullAttr)
-             && isListBlock b = b : commentSep : CodeBlock attr x :
-                                fixBlocks rest
+       | (not (variant == Commonmark ||
+               isEnabled Ext_backtick_code_blocks opts ||
+                 isEnabled Ext_fenced_code_blocks opts) ||
+              attr == nullAttr)
+            && isListBlock b
+              = b : commentSep : CodeBlock attr x : fixBlocks rest
       fixBlocks (b1@(BulletList _) : b2@(BulletList _) : bs) =
            b1 : commentSep : fixBlocks (b2:bs)
       fixBlocks (b1@(OrderedList _ _) : b2@(OrderedList _ _) : bs) =