TypeError: Cannot read property 'parent' of undefined at isTypeAssertion

[minestarks]

Repo Name: https://github.com/caolan/async.git

Revision: 62cb8ea8ce3e3b4b66cf9abc66153d2d7255985b

TypeScript Version: 4.0.0-dev.20200507

Commands:

{"response":[],"responseRequired":true}
{"command":"quickinfo-full","arguments":{"position":2047,"file":"D:/local/Temp/fuzzerWorkingDir/async/lib/cargoQueue.js"},"type":"request","seq":42}
{"responseRequired":true}
{"command":"quickinfo-full","arguments":{"position":1071,"file":"D:/local/Temp/fuzzerWorkingDir/async/lib/cargoQueue.js"},"type":"request","seq":42}
{"responseRequired":true}
{"command":"references-full","arguments":{"position":2047,"file":"D:/local/Temp/fuzzerWorkingDir/async/lib/cargoQueue.js"},"type":"request","seq":42}
{"response":[],"responseRequired":true}
{"command":"references-full","arguments":{"position":1071,"file":"D:/local/Temp/fuzzerWorkingDir/async/lib/cargoQueue.js"},"type":"request","seq":42}
{"response":[],"responseRequired":true}
{"command":"completions-full","arguments":{"position":2047,"file":"D:/local/Temp/fuzzerWorkingDir/async/lib/cargoQueue.js","includeExternalModuleExports":true,"includeInsertTextCompletions":true},"type":"request","seq":42}
(Showing last 10 of 191 commands)

Code (lib\cargoQueue.js):

   * how many tasks should be processed per round; if omitted, the default is
   * unlimited.
   * @returns {module:ControlFlow.QueueObject} A cargoQueue object to manage the tasks. Callbacks can
//                  ^
   * attached as certain properties to listen for specific events during the
   * lifecycle of the cargoQueue and inner queue.

Hash: dced6931d2c4b2cbd849ed76ef152cea
Stack trace:

TypeError: Cannot read property 'parent' of undefined
    at isTypeAssertion (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:113858:62)
    at filterGlobalCompletion (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:113833:38)
    at getGlobalCompletions (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:113804:17)
    at tryGetGlobalSymbols (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:113695:25)
    at getCompletionData (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:113489:22)
    at Object.getCompletionsAtPosition (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:112833:34)
    at Object.getCompletionsAtPosition (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:139693:35)
    at Session.getCompletions (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:149397:64)
    at Session.handlers.ts.createMapFromTemplate._a.(anonymous function) (D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:148340:61)
    at D:\home\site\wwwroot\node_modules\typescript\lib\tsserverlibrary.js:149974:88

Originally posted by @typescript-bot in https://github.com/microsoft/tsserverfuzzer/issues/227#issuecomment-625379845

[sandersn]

Couple of notes from early investigation:

1. Our code base has two isTypeAssertions. This is the one in services/completions.ts.
2. The variable that is undefined has the following comment before it:

        // The decision to provide completion depends on the contextToken, which is determined through the previousToken.
        // Note: 'previousToken' (and thus 'contextToken') can be undefined if we are the beginning of the file
        let contextToken = previousToken;

The comment says contextToken can sometimes be undefined even though its type does not contain undefined. Admittedly, this isn't at the beginning of file.

[sandersn]

Repros in a standalone file with

/**
 * @returns {module:ControlFlow}
 */
export function cargo() {
}

Repros in Code but not emacs so far. Maybe emacs isn't sending completions-full?