Forward definition requests when index lookup fails

Sources/SourceKitLSP/SourceKitServer.swift

@@ -801,22 +801,32 @@ extension SourceKitServer {
   ) {
     let symbolInfo = SymbolInfoRequest(textDocument: req.params.textDocument, position: req.params.position)
     let index = self.workspace?.index
+    // If we're unable to handle the definition request using our index, see if the
+    // language service can handle it (e.g. clangd can provide AST based definitions).
+    let resultHandler: ([Location], ResponseError?) -> Void = { (locs, error) in
+      guard locs.isEmpty else {
+        req.reply(.locations(locs))
+        return
+      }
+      let handled = languageService.definition(req)
+      guard !handled else { return }
+      if let error = error {
+        req.reply(.failure(error))
+      } else {
+        req.reply(.locations([]))
+      }
+    }
     let callback = callbackOnQueue(self.queue) { (result: Result<SymbolInfoRequest.Response, ResponseError>) in
       guard let symbols: [SymbolDetails] = result.success ?? nil, let symbol = symbols.first else {
-        let handled = languageService.definition(req)
-        guard !handled else { return }
-        if let error = result.failure {
-          req.reply(.failure(error))
-        } else {
-          req.reply(.locations([]))
-        }
+        resultHandler([], result.failure)
         return
       }
 
       let fallbackLocation = [symbol.bestLocalDeclaration].compactMap { $0 }
 
       guard let usr = symbol.usr, let index = index else {
-        return req.reply(.locations(fallbackLocation))
+        resultHandler(fallbackLocation, nil)
+        return
       }
 
       log("performing indexed jump-to-def with usr \(usr)")
@@ -842,7 +852,7 @@ extension SourceKitServer {
         )
       }
 
-      req.reply(.locations(locations.isEmpty ? fallbackLocation : locations))
+      resultHandler(locations.isEmpty ? fallbackLocation : locations, nil)
     }
     let request = Request(symbolInfo, id: req.id, clientID: ObjectIdentifier(self),
                           cancellation: req.cancellationToken, reply: callback)

Tests/INPUTS/BasicCXX/main.c

@@ -1,6 +1,6 @@
 #include /*Object:include:main*/"Object.h"
 
 int main(int argc, const char *argv[]) {
-  struct Object *obj = newObject();
+  struct /*Object:ref:main*/Object *obj = newObject();
   return obj->field;
 }

Tests/SourceKitLSPTests/SourceKitTests.swift

@@ -322,4 +322,36 @@ final class SKTests: XCTestCase {
       }
     }
   }
+
+  func testClangdGoToDefinitionWithoutIndex() throws {
+    guard let ws = try staticSourceKitTibsWorkspace(name: "BasicCXX") else { return }
+    guard ToolchainRegistry.shared.default?.clangd != nil else { return }
+
+    let refLoc = ws.testLoc("Object:ref:main")
+    let expectedDoc = ws.testLoc("Object").docIdentifier.uri
+    let refPos = Position(line: refLoc.position.line, utf16index: refLoc.utf16Column + 2)
+
+    try ws.openDocument(refLoc.url, language: .c)
+
+    let goToDefinition = DefinitionRequest(
+      textDocument: refLoc.docIdentifier, position: refPos)
+    let resp = try! ws.sk.sendSync(goToDefinition)
+
+    guard let locationsOrLinks = resp else {
+      XCTFail("No response for go-to-definition")
+      return
+    }
+    switch locationsOrLinks {
+    case .locations(let locations):
+      XCTAssert(!locations.isEmpty, "Found no locations for go-to-definition")
+      if let loc = locations.first {
+        XCTAssertEqual(loc.uri, expectedDoc)
+      }
+    case .locationLinks(let locationLinks):
+      XCTAssert(!locationLinks.isEmpty, "Found no location links for go-to-definition")
+      if let link = locationLinks.first {
+        XCTAssertEqual(link.targetUri, expectedDoc)
+      }
+    }
+  }
 }

Tests/SourceKitLSPTests/XCTestManifests.swift

@@ -143,6 +143,7 @@ extension SKTests {
     //   `swift test --generate-linuxmain`
     // to regenerate.
     static let __allTests__SKTests = [
+        ("testClangdGoToDefinitionWithoutIndex", testClangdGoToDefinitionWithoutIndex),
         ("testClangdGoToInclude", testClangdGoToInclude),
         ("testCodeCompleteSwiftTibs", testCodeCompleteSwiftTibs),
         ("testDependenciesUpdatedCXXTibs", testDependenciesUpdatedCXXTibs),