Relaxing requirement of anchorSection for headings (#403)

Author: Kyle-Ye

Sources/SwiftDocC/Model/DocumentationNode.swift

@@ -112,10 +112,10 @@ public struct DocumentationNode {
 
         for discussion in discussionSections {
             for child in discussion.content {
-                // For any H2/H3 sections found in the topic's discussion
+                // For any non-H1 Heading sections found in the topic's discussion
                 // create an `AnchorSection` and add it to `anchorSections`
                 // so we can index all anchors found in the bundle for link resolution.
-                if let heading = child as? Heading, heading.level > 1, heading.level < 4 {
+                if let heading = child as? Heading, heading.level > 1 {
                     anchorSections.append(
                         AnchorSection(reference: reference.withFragment(heading.plainText), title: heading.plainText)
                     )

Tests/SwiftDocCTests/Model/DocumentationNodeTests.swift

@@ -0,0 +1,44 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2022 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+import Foundation
+import Markdown
+@testable import SwiftDocC
+import XCTest
+
+class DocumentationNodeTests: XCTestCase {
+    func testH4AndUpAnchorSections() throws {
+        let articleSource = """
+        # Title
+
+        ## Heading2
+
+        ### Heading3
+        
+        #### Heading4
+        
+        ##### Heading5
+
+        ###### Heading6
+        """
+        
+        let article = Article(markup: Document(parsing: articleSource, options: []), metadata: nil, redirects: nil, options: [:])
+        let node = try DocumentationNode(
+            reference: ResolvedTopicReference(bundleIdentifier: "org.swift.docc", path: "/blah", sourceLanguage: .swift),
+            article: article
+        )
+        XCTAssertEqual(node.anchorSections.count, 5)
+        for (index, anchorSection) in node.anchorSections.enumerated() {
+            let expectedTitle = "Heading\(index + 2)"
+            XCTAssertEqual(anchorSection.title, expectedTitle)
+            XCTAssertEqual(anchorSection.reference, node.reference.withFragment(expectedTitle))
+        }
+    }
+}