Warn when an article would override a symbol page

Sources/SwiftDocC/Infrastructure/DocumentationContext.swift

@@ -1726,7 +1726,26 @@ public class DocumentationContext {
             }
             let reference = documentation.reference
 
-            documentationCache[reference] = documentation
+            if let existing = documentationCache[reference], existing.kind.isSymbol {
+                // By the time we get here it's already to late to fix the collision. All we can do is make the author aware of it and handle the collision deterministically.
+                // rdar://79745455 and https://github.com/swiftlang/swift-docc/issues/593 tracks fixing the root cause of this issue, avoiding the collision and allowing the article and symbol to both exist.
+                diagnosticEngine.emit(
+                    Problem(
+                        diagnostic: Diagnostic(source: article.source, severity: .warning, identifier: "org.swift.docc.articleCollisionProblem", summary: """
+                            Article '\(article.source.lastPathComponent)' (\(title)) would override \(existing.kind.name.lowercased()) '\(existing.name.description)'.
+                            """, explanation: """
+                            DocC computes unique URLs for symbols, even if they have the same name, but doesn't account for article filenames that collide with symbols because of a bug. 
+                            Until rdar://79745455 (issue #593) is fixed, DocC favors the symbol in this collision and drops the article to have deterministic behavior.
+                            """),
+                        possibleSolutions: [
+                            Solution(summary: "Rename '\(article.source.lastPathComponent)'", replacements: [ /* Renaming a file isn't something that we can represent with a replacement */ ])
+                        ]
+                    )
+                )
+                return article // Don't continue processing this article
+            } else {
+                documentationCache[reference] = documentation
+            }
 
             documentLocationMap[article.source] = reference
             let graphNode = TopicGraph.Node(reference: reference, kind: .article, source: .file(url: article.source), title: title)

Tests/SwiftDocCTests/Infrastructure/DocumentationContext/DocumentationContextTests.swift

@@ -2458,7 +2458,10 @@ let expected = """
         let curatedTopic = try XCTUnwrap(renderNode.topicSections.first?.identifiers.first)
 
         let topicReference = try XCTUnwrap(renderNode.references[curatedTopic] as? TopicRenderReference)
-        XCTAssertEqual(topicReference.title, "An article")
+
+        // FIXME: Verify that article matches are preferred for general (non-symbol) links once rdar://79745455 https://github.com/swiftlang/swift-docc/issues/593 is fixed
+        XCTAssertEqual(topicReference.title, "Wrapper")
+//        XCTAssertEqual(topicReference.title, "An article")
 
         // This test also reproduce https://github.com/swiftlang/swift-docc/issues/593
         // When that's fixed this test should also use a symbol link to curate the top-level symbol and verify that
@@ -4806,6 +4809,41 @@ let expected = """
         XCTAssertEqual(problem.diagnostic.summary, "\'NonExistingDoc\' doesn\'t exist at \'/BestBook/MyArticle\'")
     }
 
+    func testArticleCollidingWithSymbol() async throws {
+        let catalog = Folder(name: "ModuleName.docc", content: [
+            JSONFile(name: "ModuleName.symbols.json", content: makeSymbolGraph(moduleName: "ModuleName", symbols: [
+                makeSymbol(id: "some-symbol-id", kind: .class, pathComponents: ["SomeClass"]), // Collision
+            ])),
+
+            TextFile(name: "SomeClass.md", utf8Content: """
+            # Some article
+
+            This article has the same reference as the symbol. One will override the other. 
+            We should at least warn about that.
+            """),
+        ])
+
+        let (_, context) = try await loadBundle(catalog: catalog)
+        let moduleReference = try XCTUnwrap(context.soleRootModuleReference)
+        let collidingPageReference = moduleReference.appendingPath("SomeClass")
+
+        XCTAssertEqual(
+            Set(context.knownPages), [moduleReference, collidingPageReference],
+            "Ideally there should be 3 pages here but because of rdar://79745455 (issue #593) there isn't" // https://github.com/swiftlang/swift-docc/issues/593
+        )
+
+        let node = try context.entity(with: collidingPageReference)
+        XCTAssert(node.kind.isSymbol, "Given #593 / rdar://79745455 we should deterministically prioritize the symbol over the article")
+
+        XCTAssertEqual(context.problems.map(\.diagnostic.summary), [
+            "Article 'SomeClass.md' (Some article) would override class 'SomeClass'."
+        ])
+
+        let problem = try XCTUnwrap(context.problems.first)
+        let solution = try XCTUnwrap(problem.possibleSolutions.first)
+        XCTAssertEqual(solution.summary, "Rename 'SomeClass.md'")
+    }
+
     func testContextRecognizesOverloads() async throws {
         enableFeatureFlag(\.isExperimentalOverloadedSymbolPresentationEnabled)
 

Tests/SwiftDocCTests/Infrastructure/PathHierarchyTests.swift

@@ -1239,8 +1239,11 @@ class PathHierarchyTests: XCTestCase {
         // The added article above has the same path as an existing symbol in the this module.
         let symbolNode = try tree.findNode(path: "/MixedLanguageFramework/Bar", onlyFindSymbols: true)
         XCTAssertNotNil(symbolNode.symbol, "Symbol link finds the symbol")
+
         let articleNode = try tree.findNode(path: "/MixedLanguageFramework/Bar", onlyFindSymbols: false)
-        XCTAssertNil(articleNode.symbol, "General documentation link find the article")
+        XCTAssertNotNil(articleNode.symbol, "This should be an article but can't be because of rdar://79745455")
+        // FIXME: Verify that article matches are preferred for general (non-symbol) links once  https://github.com/swiftlang/swift-docc/issues/593 is fixed
+//        XCTAssertNil(articleNode.symbol, "General documentation link find the article")
     }
 
     func testArticleSelfAnchorLinks() async throws {
@@ -2491,13 +2494,17 @@ class PathHierarchyTests: XCTestCase {
             // Links to non-symbols can use only the file name, without specifying the module or catalog name.
             let articleID = try tree.find(path: "Wrapper", onlyFindSymbols: false)
             let articleMatch = try XCTUnwrap(tree.lookup[articleID])
-            XCTAssertNil(articleMatch.symbol, "Should have found the article")
+            XCTAssertNotNil(articleMatch.symbol, "This should be an article but can't be because of rdar://79745455")
+            // FIXME: Verify that article matches are preferred for general (non-symbol) links once rdar://79745455 https://github.com/swiftlang/swift-docc/issues/593 is fixed
+//            XCTAssertNil(articleMatch.symbol, "Should have found the article")
         }
         do {
             // Links to non-symbols can also use module-relative links.
             let articleID = try tree.find(path: "/Something/Wrapper", onlyFindSymbols: false)
             let articleMatch = try XCTUnwrap(tree.lookup[articleID])
-            XCTAssertNil(articleMatch.symbol, "Should have found the article")
+            XCTAssertNotNil(articleMatch.symbol, "This should be an article but can't be because of rdar://79745455")
+            // FIXME: Verify that article matches are preferred for general (non-symbol) links once rdar://79745455 https://github.com/swiftlang/swift-docc/issues/593 is fixed
+//            XCTAssertNil(articleMatch.symbol, "Should have found the article")
         }
         // Symbols can only use absolute links or be found relative to another page.
         let symbolID = try tree.find(path: "/Something/Wrapper", onlyFindSymbols: true)