Only report a single item in prepareCallHierarchy

Returning multiple doesn't make sense because they will all have the same USR (because we query them by USR) and will thus expand to the exact same call hierarchy.

Also, VS Code doesn't seem to like multiple call hiearchy items being returned and fails to display any results if they are, failing with `Cannot read properties of undefined (reading 'map')`.

rdar://118957893

Author: ahoppen

Sources/SourceKitLSP/SourceKitServer.swift

@@ -1752,10 +1752,7 @@ extension SourceKitServer {
     }
     guard let usr = symbol.usr else { return [] }
     logger.info("performing indexed jump-to-def with usr \(usr)")
-    var occurrences = index.occurrences(ofUSR: usr, roles: [.definition])
-    if occurrences.isEmpty {
-      occurrences = index.occurrences(ofUSR: usr, roles: [.declaration])
-    }
+    var occurrences = index.definitionOrDeclarationOccurances(ofUSR: usr)
     if symbol.isDynamic ?? true {
       lazy var transitiveReceiverUsrs: [String]? = {
         if let receiverUsrs = symbol.receiverUsrs {
@@ -1963,17 +1960,23 @@ extension SourceKitServer {
     }
     // For call hierarchy preparation we only locate the definition
     guard let usr = symbol.usr else { return nil }
-    return index.occurrences(ofUSR: usr, roles: [.definition, .declaration])
-      .compactMap { info -> CallHierarchyItem? in
-        guard let location = indexToLSPLocation(info.location) else {
-          return nil
-        }
-        return self.indexToLSPCallHierarchyItem(
-          symbol: info.symbol,
-          moduleName: info.location.moduleName,
-          location: location
-        )
-      }
+
+    // Only return a single call hierarchy item. Returning multiple doesn't make sense because they will all have the
+    // same USR (because we query them by USR) and will thus expand to the exact same call hierarchy.
+    // Also, VS Code doesn't seem to like multiple call hiearchy items being returned and fails to display any results
+    // if they are, failing with `Cannot read properties of undefined (reading 'map')`.
+    guard let definition = index.definitionOrDeclarationOccurrences(ofUSR: usr).first else {
+      return nil
+    }
+    guard let location = indexToLSPLocation(definition.location) else {
+      return nil
+    }
+    let callHierachyItem = self.indexToLSPCallHierarchyItem(
+      symbol: definition.symbol,
+      moduleName: definition.location.moduleName,
+      location: location
+    )
+    return [callHierachyItem]
   }
 
   /// Extracts our implementation-specific data about a call hierarchy
@@ -2255,6 +2258,18 @@ private let maxWorkspaceSymbolResults = 4096
 
 public typealias Diagnostic = LanguageServerProtocol.Diagnostic
 
+fileprivate extension IndexStoreDB {
+  /// If there are any definition occurrences of the given USR, return these.
+  /// Otherwise return declaration occurrences.
+  func definitionOrDeclarationOccurrences(ofUSR usr: String) -> [SymbolOccurrence] {
+    let definitions = occurrences(ofUSR: usr, roles: [.definition])
+    if !definitions.isEmpty {
+      return definitions
+    }
+    return occurrences(ofUSR: usr, roles: [.declaration])
+  }
+}
+
 extension IndexSymbolKind {
   func asLspSymbolKind() -> SymbolKind {
     switch self {

Tests/SourceKitLSPTests/CallHierarchyTests.swift

@@ -183,4 +183,45 @@ final class CallHierarchyTests: XCTestCase {
       ]
     )
   }
+
+  func testReportSingleItemInPrepareCallHierarchy() async throws {
+    let ws = try await SwiftPMTestWorkspace(
+      files: [
+        "MyLibrary/include/lib.h": """
+        struct FilePathIndex {
+          void 1️⃣foo();
+        };
+        """,
+        "MyLibrary/lib.cpp": """
+        #include "lib.h"
+        void FilePathIndex::2️⃣foo() {}
+        """,
+      ],
+      build: true
+    )
+    let (uri, positions) = try ws.openDocument("lib.h", language: .cpp)
+    let result = try await ws.testClient.send(
+      CallHierarchyPrepareRequest(textDocument: TextDocumentIdentifier(uri), position: positions["1️⃣"])
+    )
+
+    // Test that we don't provide both the definition in .cpp and the declaration on .h
+    XCTAssertEqual(
+      result,
+      [
+        CallHierarchyItem(
+          name: "foo",
+          kind: .method,
+          tags: nil,
+          detail: "",
+          uri: try ws.uri(for: "lib.cpp"),
+          range: try Range(ws.position(of: "2️⃣", in: "lib.cpp")),
+          selectionRange: try Range(ws.position(of: "2️⃣", in: "lib.cpp")),
+          data: LSPAny.dictionary([
+            "usr": .string("c:@S@FilePathIndex@F@foo#"),
+            "uri": .string(try ws.uri(for: "lib.cpp").stringValue),
+          ])
+        )
+      ]
+    )
+  }
 }