Problem with fenced_code_attributes with commonmark

[wch]

I was trying to use commonmark+fenced_code_attributes and gfm+fenced_code_attributes with one of the examples from the manual, but it didn't work -- the attributes like .haskell would get lost in the output:

cat << EOF | pandoc -f commonmark+fenced_code_attributes -t native
~~~~ {#mycode .haskell .numberLines startFrom="100"}
qsort []     = []
qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++
               qsort (filter (>= x) xs)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
EOF

The result:

[CodeBlock ("",["{#mycode"],[]) "qsort []     = []\nqsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++\n               qsort (filter (>= x) xs)"]

I ended up spending an embarrassingly long time trying to figure out what was wrong, and finally discovered that it was because commonmark and gfm have -fenced_code_attributes:

$ pandoc --list-extensions=commonmark
-ascii_identifiers
-autolink_bare_uris
-bracketed_spans
-definition_lists
-east_asian_line_breaks
-emoji
-fancy_lists
-fenced_code_attributes
-fenced_divs
-footnotes
-gfm_auto_identifiers
-hard_line_breaks
-implicit_figures
-implicit_header_references
-pipe_tables
-raw_attribute
+raw_html
-raw_tex
-smart
-strikeout
-subscript
-superscript
-task_lists
-tex_math_dollars
-attributes
-sourcepos

So when I used +fenced_code_attributes, it was silently being ignored. It would be much friendlier behavior to either print an error message in this situation, or allow +fenced_code_attributes to override the setting in commonmark.

[jgm]

When this prints -fenced_code_attributes, it means that this extension will affect the format, and that it is set to disabled by default. So +fenced_code_attributes should work.
Looks like +fenced_code_attributes+attributes does work.
(And also +attributes by itself). I need to look into this.