Fix issue that causes type hierarchy to not work for clang files

Sources/SourceKitLSP/SourceKitLSPServer.swift

@@ -2464,21 +2464,32 @@ extension SourceKitLSPServer {
     guard let index = await workspaceForDocument(uri: req.textDocument.uri)?.index(checkedFor: .deletedFiles) else {
       return nil
     }
-    let usrs =
-      symbols
-      .filter {
-        // Only include references to type. For example, we don't want to find the type hierarchy of a constructor when
-        // starting the type hierarchy on `Foo()``.
-        switch $0.kind {
-        case .class, .enum, .interface, .struct: return true
-        default: return false
-        }
+    let usrs = symbols.filter {
+      // Only include references to type. For example, we don't want to find the type hierarchy of a constructor when
+      // starting the type hierarchy on `Foo()`.
+      // Consider a symbol a class if its kind is `nil`, eg. for a symbol returned by clang's SymbolInfo, which
+      // doesn't support the `kind` field.
+      switch $0.kind {
+      case .class, .enum, .interface, .struct, nil: return true
+      default: return false
       }
-      .compactMap(\.usr)
+    }.compactMap(\.usr)
+
     let typeHierarchyItems = usrs.compactMap { (usr) -> TypeHierarchyItem? in
       guard let info = index.primaryDefinitionOrDeclarationOccurrence(ofUSR: usr) else {
         return nil
       }
+      // Filter symbols based on their kind in the index since the filter on the symbol info response might have
+      // returned `nil` for the kind, preventing us from doing any filtering there.
+      switch info.symbol.kind {
+      case .unknown, .macro, .function, .variable, .field, .enumConstant, .instanceMethod, .classMethod, .staticMethod,
+        .instanceProperty, .classProperty, .staticProperty, .constructor, .destructor, .conversionFunction, .parameter,
+        .concept, .commentTag:
+        return nil
+      case .module, .namespace, .namespaceAlias, .enum, .struct, .class, .protocol, .extension, .union, .typealias,
+        .using:
+        break
+      }
       return self.indexToLSPTypeHierarchyItem(
         definition: info,
         moduleName: info.location.moduleName,

Tests/SourceKitLSPTests/TypeHierarchyTests.swift

@@ -12,6 +12,7 @@
 
 import LanguageServerProtocol
 import SKTestSupport
+import SwiftExtensions
 import TSCBasic
 import XCTest
 
@@ -248,6 +249,46 @@ final class TypeHierarchyTests: XCTestCase {
       ]
     )
   }
+
+  func testClangTypeHierarchy() async throws {
+    let project = try await SwiftPMTestProject(
+      files: [
+        "MyLibrary/include/empty.h": "",
+        "MyLibrary/Test.cpp": """
+        class Foo {};
+
+        class Bar: 1️⃣Foo {};
+        """,
+      ],
+      enableBackgroundIndexing: true
+    )
+    let (uri, positions) = try project.openDocument("Test.cpp", language: .cpp)
+    let response = try await project.testClient.send(
+      TypeHierarchyPrepareRequest(textDocument: TextDocumentIdentifier(uri), position: positions["1️⃣"])
+    )
+    XCTAssertEqual(response?.count, 1)
+  }
+
+  func testClangTypeHierarchyInitiatedFromFunction() async throws {
+    let project = try await SwiftPMTestProject(
+      files: [
+        "MyLibrary/include/empty.h": "",
+        "MyLibrary/Test.cpp": """
+        void hello() {}
+
+        void test() {
+          1️⃣hello();
+        }
+        """,
+      ],
+      enableBackgroundIndexing: true
+    )
+    let (uri, positions) = try project.openDocument("Test.cpp", language: .cpp)
+    let response = try await project.testClient.send(
+      TypeHierarchyPrepareRequest(textDocument: TextDocumentIdentifier(uri), position: positions["1️⃣"])
+    )
+    XCTAssertNil(response)
+  }
 }
 
 // MARK: - Utilities