Add a giant TODO comment listing the remaining Markdown cases we don't handle well.

Also add test methods that currently illustrate the incorrect handling. As we fix each issue, we can delete it from the comment and ensure that its test method produces the expected result.

PiperOrigin-RevId: 897161602

Author: eamonnmcmanus

core/src/test/java/com/google/googlejavaformat/java/JavadocFormattingTest.java

@@ -1775,6 +1775,187 @@ class Test {}
     doFormatTest(input, expected);
   }
 
+  @Test
+  public void markdownBackslashes() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// \\<br> is not a break.
+        /// \\&#42; is not an HTML entity.
+        /// foo\\
+        /// bar
+        class Test {}
+        """;
+    // TODO: the <br> should not cause a line break, and the end-of-line backslash should.
+    // I don't think anything changes if we do or do not respect the \& backslash.
+    String expected =
+        """
+        /// \\<br>
+        /// is not a break. \\&#42; is not an HTML entity. foo\\ bar
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void markdownThematicBreaks() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// foo
+        /// ***
+        /// bar
+        class Test {}
+        """;
+    // TODO: the line break before `***` should be preserved.
+    // It's OK to introduce a blank line before `bar` since it is a new paragraph.
+    String expected =
+        """
+        /// foo ***
+        ///
+        /// bar
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void markdownSetextHeadings() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// Heading
+        /// =======
+        /// Phoebe B. Peabody-Beebe
+        ///
+        /// Subheading
+        /// ----------
+        class Test {}
+        """;
+    // TODO: the line breaks before the lines of repeated characters should be preserved.
+    //    Or, we could rewrite this style of heading as `# Heading`.
+    String expected =
+        """
+        /// Heading =======
+        ///
+        /// Phoebe B. Peabody-Beebe
+        ///
+        /// Subheading ----------
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void markdownIndentedCodeBlocks() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        ///     code block
+        ///     is indented
+        class Test {}
+        """;
+    // TODO: the evil indented code block should be preserved.
+    String expected =
+        """
+        /// code block is indented
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void markdownLinkReferenceDefinitions() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// [foo]
+        /// [foo]: /url "title"
+        class Test {}
+        """;
+    String expected =
+        """
+        /// [foo] [foo]: /url "title"
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void markdownLooseLists() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// - item 1
+        ///
+        /// - item 2
+        class Test {}
+        """;
+    // TODO: the line break between items should be preserved, and there should not be a blank line
+    //   before the list.
+    String expected =
+        """
+        ///
+        /// - item 1
+        /// - item 2
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void markdownBlockQuotes() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// > foo
+        /// > bar
+        class Test {}
+        """;
+    // TODO: the block quote should be preserved, and ideally bar would be joined to foo.
+    String expected =
+        """
+        /// >
+        ///
+        /// foo > bar
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void markdownCodeSpans() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// `<ul>` should not trigger list handling.
+        class Test {}
+        """;
+    // TODO: the <ul> should not be recognized as a list, so `<ul>` should be preserved.
+    // TODO: test that text with `...` is subject to line wrapping, including joining short lines.
+    String expected =
+        """
+        /// `<ul>
+        ///   ` should not trigger list handling.
+        class Test {}
+        """;
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void markdownAutolinks() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+        """
+        /// <http://example.com> should be preserved.
+        class Test {}
+        """;
+    // TODO: find a test case that will break if autolinks are not handled correctly.
+    // Probably something like: <http://{@code>this should not be handled like a code span}
+    String expected = input;
+    doFormatTest(input, expected);
+  }
+
   // TODO: b/346668798 - Test the following Markdown constructs, and make the tests work as needed.
   // We can assume that the CommonMark parser correctly handles Markdown, so the question is whether
   // they are subsequently mishandled by our formatting logic. So for example the CommonMark parser
@@ -1821,7 +2002,10 @@ class Test {}
   //   inside a list. https://spec.commonmark.org/0.31.2/#block-quotes
   //
   // - Code spans
-  //   `<ul>` should not trigger list handling. https://spec.commonmark.org/0.31.2/#code-spans
+  //   `<ul>` should not trigger list handling.
+  //   Text within `...` should still be subject to line wrapping, both splitting long lines and
+  //   joining short lines. https://spec.commonmark.org/0.31.2/#code-spans
+  //
   //
   // - Autolinks
   //   <http://example.com> should be preserved. https://spec.commonmark.org/0.31.2/#autolink