Instead of returning an empty array in prepareTypeHierarchy, sourcekit-lsp should return nil

Eg. when requesting type hierarchy of a class when the project hasn’t been built, sourcekit-lsp returns an empty array. That causes VS Code to fail with very ambiguous error messages
- MISSING provider
- Cannot read properties of null (reading 'kind')

To work around this, instead of returning an empty array, return `nil` instead.

Author: ahoppen

Sources/SourceKitLSP/SourceKitLSPServer.swift

@@ -2291,6 +2291,13 @@ extension SourceKitLSPServer {
     }
     .sorted(by: { $0.name < $1.name })
 
+    if typeHierarchyItems.isEmpty {
+      // When returning an empty array, VS Code fails with the following two errors. Returning `nil` works around those
+      // VS Code-internal errors showing up
+      //  - MISSING provider
+      //  - Cannot read properties of null (reading 'kind')
+      return nil
+    }
     // Ideally, we should show multiple symbols. But VS Code fails to display type hierarchies with multiple root items,
     // failing with `Cannot read properties of undefined (reading 'map')`. Pick the first one.
     return Array(typeHierarchyItems.prefix(1))

Tests/SourceKitLSPTests/TypeHierarchyTests.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import ISDBTestSupport
+import LSPTestSupport
 import LanguageServerProtocol
 import SKTestSupport
 import TSCBasic
@@ -185,6 +186,28 @@ final class TypeHierarchyTests: XCTestCase {
       ]
     )
   }
+
+  func testTypeHierarchyForFileWithoutIndex() async throws {
+    let project = try await SwiftPMTestProject(files: [
+      "Test.swift": """
+      class MyClass {}
+      class 1️⃣MySubclass: MyClass {}
+
+      let x = MySubclass()
+      """
+    ])
+
+    let (uri, positions) = try project.openDocument("Test.swift")
+
+    // We don't have the type hierarchy because the file hasn't been indexed yet. Returning an empty array makes VS Code
+    // fail with the following two errors, so we should return `nil` instead.
+    //  - MISSING provider
+    //  - Cannot read properties of null (reading 'kind')
+    let response = try await project.testClient.send(
+      TypeHierarchyPrepareRequest(textDocument: TextDocumentIdentifier(uri), position: positions["1️⃣"])
+    )
+    XCTAssertNil(response)
+  }
 }
 
 // MARK: - Utilities