Fix error on emphasis around whitespace character reference (#492)

docs/users/changelog.md

@@ -18,6 +18,9 @@ Note that there is currently no guarantee for a stable Markdown formatting style
 - Fixed
   - Read UTF-8 from standard input on all systems.
     Thank you, [Christopher Prohm](https://github.com/chmp), for the PR.
+  - No longer error on emphasis whose content is a whitespace character
+    reference (e.g. `* *`). The boundary whitespace is now kept in
+    character reference form so the emphasis markers remain functional.
 
 ## 0.7.22
 

src/mdformat/renderer/_context.py

@@ -289,12 +289,22 @@ def link(node: RenderTreeNode, context: RenderContext) -> str:
 
 def em(node: RenderTreeNode, context: RenderContext) -> str:
     text = make_render_children(separator="")(node, context)
+    # Emphasis can not begin or end in Unicode whitespace, so if it does
+    # (e.g. the whitespace came from a character reference like ` `),
+    # re-decimalify the boundary whitespace to keep the markers functional.
+    text = decimalify_leading(codepoints.UNICODE_WHITESPACE, text)
+    text = decimalify_trailing(codepoints.UNICODE_WHITESPACE, text)
     indicator = node.markup
     return indicator + text + indicator
 
 
 def strong(node: RenderTreeNode, context: RenderContext) -> str:
     text = make_render_children(separator="")(node, context)
+    # Emphasis can not begin or end in Unicode whitespace, so if it does
+    # (e.g. the whitespace came from a character reference like ` `),
+    # re-decimalify the boundary whitespace to keep the markers functional.
+    text = decimalify_leading(codepoints.UNICODE_WHITESPACE, text)
+    text = decimalify_trailing(codepoints.UNICODE_WHITESPACE, text)
     indicator = node.markup
     return indicator + text + indicator
 

tests/data/default_style.md

@@ -530,3 +530,17 @@ Reduce to a space	 	here.
 
 Reduce to a space here.
 .
+
+emphasis around whitespace character reference (issue 492)
+.
+* *
+.
+* *
+.
+
+strong emphasis around named whitespace character reference
+.
+** **
+.
+** **
+.