Fix regression where DocC no longer emits warnings for unresolved links. (#774)

rdar://119603319

Emit warnings for unresolved links inside articles.

Regression was presumably introduced in da16aa9. When the node is an article we don’t want to check the offset because is not a source file. Before the fix, the logic was removing all problems because it wouldn’t contain an offset, falling through the else condition.

Added test to make sure that problems are generated when an article or symbol extension file contains unresolved links.

Author: sofiaromorales

Sources/SwiftDocC/Infrastructure/DocumentationContext.swift

@@ -599,7 +599,7 @@ public class DocumentationContext: DocumentationContextDataProviderDelegate {
 
                     var problems = resolver.problems
 
-                    if case .sourceCode(_, let offset) = doc.source {
+                    if case .sourceCode(_, let offset) = doc.source, documentationNode.kind.isSymbol {
                         // Offset all problem ranges by the start location of the
                         // source comment in the context of the complete file.
                         if let docRange = offset {

Tests/SwiftDocCTests/Infrastructure/DocumentationContext/DocumentationContextTests.swift

@@ -4030,6 +4030,55 @@ let expected = """
         let range = try XCTUnwrap(problem.diagnostic.range)
         XCTAssertEqual(start..<end, range)
     }
+    
+    func testUnresolvedLinkWarnings() throws {
+        var (_, _, context) = try testBundleAndContext(copying: "TestBundle") { url in
+            let extensionFile = """
+            # ``SideKit``
+
+            myFunction abstract
+
+            ## Overview
+
+            This is unresolvable: <doc:Does-Not-Exist>.
+            
+            ## Topics
+            
+            - <doc:NonExistingDoc>
+
+            """
+            let fileURL = url.appendingPathComponent("documentation").appendingPathComponent("myFunction.md")
+            try extensionFile.write(to: fileURL, atomically: true, encoding: .utf8)
+        }
+        var problems = context.diagnosticEngine.problems
+        var linkResolutionProblems = problems.filter { $0.diagnostic.source?.relativePath.hasSuffix("myFunction.md") == true }
+        XCTAssertEqual(linkResolutionProblems.count, 3)
+        var problem = try XCTUnwrap(linkResolutionProblems.last)
+        XCTAssertEqual(problem.diagnostic.summary, "\'NonExistingDoc\' doesn\'t exist at \'/SideKit\'")
+        (_, _, context) = try testBundleAndContext(copying: "BookLikeContent") { url in
+            let extensionFile = """
+            # My Article
+
+            Abstract
+
+            ## Overview
+
+            Overview
+            
+            ## Topics
+            
+            - <doc:NonExistingDoc>
+
+            """
+            let fileURL = url.appendingPathComponent("MyArticle.md")
+            try extensionFile.write(to: fileURL, atomically: true, encoding: .utf8)
+        }
+        problems = context.diagnosticEngine.problems
+        linkResolutionProblems = problems.filter { $0.diagnostic.source?.relativePath.hasSuffix("MyArticle.md") == true }
+        XCTAssertEqual(linkResolutionProblems.count, 1)
+        problem = try XCTUnwrap(linkResolutionProblems.last)
+        XCTAssertEqual(problem.diagnostic.summary, "\'NonExistingDoc\' doesn\'t exist at \'/BestBook/MyArticle\'")
+    }
 }
 
 func assertEqualDumps(_ lhs: String, _ rhs: String, file: StaticString = #file, line: UInt = #line) {