From f482982c471b9fbf85b144a9fc723bb58a2df12b Mon Sep 17 00:00:00 2001
From: Alex Hoppen <ahoppen@apple.com>
Date: Tue, 16 Apr 2024 11:25:39 -0700
Subject: [PATCH] Instead of returning an empty array in
 `prepareTypeHierarchy`, sourcekit-lsp should return `nil`
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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.

rdar://126228814
---
 Sources/SourceKitLSP/SourceKitLSPServer.swift |  7 ++++++
 .../TypeHierarchyTests.swift                  | 23 +++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/Sources/SourceKitLSP/SourceKitLSPServer.swift b/Sources/SourceKitLSP/SourceKitLSPServer.swift
index fa1979c9f..bfb6345eb 100644
--- a/Sources/SourceKitLSP/SourceKitLSPServer.swift
+++ b/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))
diff --git a/Tests/SourceKitLSPTests/TypeHierarchyTests.swift b/Tests/SourceKitLSPTests/TypeHierarchyTests.swift
index ed8a6e211..614fa9265 100644
--- a/Tests/SourceKitLSPTests/TypeHierarchyTests.swift
+++ b/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