attr: fix concat of class attrs on union

src/attr.rs

@@ -145,7 +145,7 @@ impl<'s> Attributes<'s> {
         if let Some(attrs0) = &mut self.0 {
             if let Some(mut attrs1) = other.0 {
                 for (key, val) in attrs1.drain(..) {
-                    if !attrs0.iter().any(|(k, _)| *k == key) {
+                    if key == "class" || !attrs0.iter().any(|(k, _)| *k == key) {
                         attrs0.push((key, val));
                     }
                 }

src/lib.rs

@@ -1776,6 +1776,37 @@ mod test {
         );
     }
 
+    #[test]
+    fn link_reference_attrs_class() {
+        test_parse!(
+            concat!(
+                "[text][tag]{.link}\n",
+                "\n",
+                "{.def}\n",
+                "[tag]: url\n",
+                "para\n",
+            ),
+            Start(Paragraph, Attributes::new()),
+            Start(
+                Link("url".into(), LinkType::Span(SpanLinkType::Reference)),
+                [("class", "link"), ("class", "def")].into_iter().collect(),
+            ),
+            Str("text".into()),
+            End(Link("url".into(), LinkType::Span(SpanLinkType::Reference))),
+            End(Paragraph),
+            Blankline,
+            Start(
+                LinkDefinition { label: "tag" },
+                [("class", "def")].into_iter().collect()
+            ),
+            Str("url".into()),
+            End(LinkDefinition { label: "tag" }),
+            Start(Paragraph, Attributes::new()),
+            Str("para".into()),
+            End(Paragraph),
+        );
+    }
+
     #[test]
     fn autolink() {
         test_parse!(