cross references should work with distill_article as base format

[cderv]

This should be working IMO

---
title: "Untitled"
date: "`r Sys.Date()`"
output:
  bookdown::html_document2:
    base_format: distill::distill_article
    pandoc_args: ["--number-sections"]
---

# heading 1 {#myheading1}

See section \@ref(myheading1)

Reported in rstudio/distill#287

Currently it is not because distill will generate this HTML

"<h1 data-number=\"1\" id=\"myheading1\"><span class=\"header-section-number\">1</span> heading 1</h1>"

which bookdown regex does not currently support. This should be solve when we'll change how to solve reference (without regex) but for now I guess just modifying the regex will work.

bookdown/R/html.R

Lines 706 to 719 in 0098721

	parse_section_labels = function(content) {
	arry = character()
	sec_ids = '^<div id="([^"]+)" class="section .+$'
	for (i in grep(sec_num, content)) {
	if (!grepl(sec_ids, content[i - 1])) next # no section id
	# extract section number and id, store in the array
	arry = c(arry, setNames(
	sub(sec_num, '\\1', content[i]),
	sub(sec_ids, '\\1', content[i - 1])
	))
	}
	write_ref_keys(names(arry))
	arry
	}

bookdown/R/html.R

Line 703 in 0098721

sec_num = '^<h[1-6]><span class="header-section-number">([.A-Z0-9]+)</span>.+</h[1-6]>(\n</div>)?$'

[cderv]

The issue is deeper than I thought.

Maybe changing the regex would work but bookdown expect HTML4 I believe and distill produces HTML5. Hence some content is different.

For example, bookdown expect sections so --sections-div needs to be activated (which is not in distill) but this still does not work

---
title: "Untitled"
date: "`r Sys.Date()`"
output:
  bookdown::html_document2:
    base_format: distill::distill_article
    pandoc_args: ["--number-sections", "--section-divs"]
---

# heading 1 {#myheading1}

See section \@ref(myheading1)

because this will produce

<section id="myheading1" class="level1" data-number="1">

and not a div with class section.

Should we try to support this ?

I believe this is another case for making referencing possible from rmarkdown directly.