Extract excerpt from RDoc::Markup::Document (page documents) correctly

Fixes https://bugs.ruby-lang.org/issues/20862

lib/rdoc/generator/darkfish.rb

@@ -782,7 +782,22 @@ def template_for file, page = true, klass = ERB
 
   # Returns an excerpt of the content for usage in meta description tags
   def excerpt(content)
-    text = content.is_a?(RDoc::Comment) ? content.text : content
+    text = case content
+    when RDoc::Comment
+      content.text
+    when RDoc::Markup::Document
+      first_part = content.parts.first
+
+      if first_part.respond_to?(:text)
+        first_part.text
+      else
+        # TODO: We may want to find a way to proceed to the following parts
+        # instead
+        return ""
+      end
+    else
+      content
+    end
 
     # Match from a capital letter to the first period, discarding any links, so
     # that we don't end up matching badges in the README

test/rdoc/test_rdoc_generator_darkfish.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 require_relative 'helper'
 
-class TestRDocGeneratorDarkfish < RDoc::TestCase
+class RDocGeneratorDarkfishTest < RDoc::TestCase
 
   def setup
     super
@@ -367,6 +367,48 @@ def test_meta_tags_for_pages
     )
   end
 
+  def test_meta_tags_for_pages
+    top_level = @store.add_file("MyPage", parser: RDoc::Parser::Simple)
+    top_level.comment = RDoc::Markup::Document.new(RDoc::Markup::Paragraph.new('this is a comment'))
+
+    @g.generate
+
+    content = File.binread("MyPage.html")
+    assert_include(content, '<meta name="keywords" content="ruby,documentation,MyPage">')
+    assert_include(
+      content,
+      '<meta name="description" content="MyPage: this is a comment">',
+    )
+  end
+
+  def test_meta_tags_for_pages_with_non_text_parts
+    top_level = @store.add_file("MyPage", parser: RDoc::Parser::Simple)
+    top_level.comment = RDoc::Markup::Document.new(RDoc::Markup::BlankLine.new)
+
+    @g.generate
+
+    content = File.binread("MyPage.html")
+    assert_include(content, '<meta name="keywords" content="ruby,documentation,MyPage">')
+    assert_include(
+      content,
+      '<meta name="description" content="MyPage: ">',
+    )
+  end
+
+  def test_meta_tags_for_empty_document
+    top_level = @store.add_file("MyPage", parser: RDoc::Parser::Simple)
+    top_level.comment = RDoc::Markup::Document.new
+
+    @g.generate
+
+    content = File.binread("MyPage.html")
+    assert_include(content, '<meta name="keywords" content="ruby,documentation,MyPage">')
+    assert_include(
+      content,
+      '<meta name="description" content="MyPage: ">',
+    )
+  end
+
   ##
   # Asserts that +filename+ has a link count greater than 1 if hard links to
   # @tmpdir are supported.