Language service crashes when requesting type of mixin class from JS

[sandersn]

// @ts-check
/**
 * @template {new (...args: any[]) => any} T
 * @param {T} S
 */
function mix(S) {
    return class extends S {
        f() { return 0; }
    }
};

class Base {}
const Mixed/**/ = mix(Base);

Request the type of Mixed from the language service.

Expected behavior:

const Mixed: {
   new (...args: any[]): mix<typeof Base>.(Anonymous class);
  prototype: max<any>.(Anonymous class);
} & typeof Base

(Just like Typescript)

Actual behavior:

Crash:

Error processing request. Cannot read property 'flags' of undefined
TypeError: Cannot read property 'flags' of undefined
    at lookupSymbolChain (/home/nathansa/ts/built/local/tsserver.js:32817:46)
    at symbolToTypeNode (/home/nathansa/ts/built/local/tsserver.js:32930:29)
    at typeReferenceToTypeNode (/home/nathansa/ts/built/local/tsserver.js:32502:47)
    at typeToTypeNodeHelper (/home/nathansa/ts/built/local/tsserver.js:32265:28)
    at signatureToSignatureDeclarationHelper (/home/nathansa/ts/built/local/tsserver.js:32738:52)
    at createTypeNodesFromResolvedType (/home/nathansa/ts/built/local/tsserver.js:32571:43)
    at createTypeNodeFromObjectType (/home/nathansa/ts/built/local/tsserver.js:32441:35)
    at createAnonymousTypeNode (/home/nathansa/ts/built/local/tsserver.js:32394:42)
    at typeToTypeNodeHelper (/home/nathansa/ts/built/local/tsserver.js:32310:28)
    at mapToTypeNodes (/home/nathansa/ts/built/local/tsserver.js:32688:40)
    at typeToTypeNodeHelper (/home/nathansa/ts/built/local/tsserver.js:32295:37)
    at /home/nathansa/ts/built/local/tsserver.js:32117:106
    at withContext (/home/nathansa/ts/built/local/tsserver.js:32152:37)
    at Object.typeToTypeNode (/home/nathansa/ts/built/local/tsserver.js:32117:28)
    at typeToString (/home/nathansa/ts/built/local/tsserver.js:32096:40)
    at Object.writeType (/home/nathansa/ts/built/local/tsserver.js:29503:24)
    at /home/nathansa/ts/built/local/tsserver.js:90305:25
    at mapToDisplayParts (/home/nathansa/ts/built/local/tsserver.js:90294:13)
    at Object.typeToDisplayParts (/home/nathansa/ts/built/local/tsserver.js:90304:16)
    at Object.getSymbolDisplayPartsDocumentationAndSymbolKind (/home/nathansa/ts/built/local/tsserver.js:100495:62)
    at /home/nathansa/ts/built/local/tsserver.js:112087:41
    at Object.runWithCancellationToken (/home/nathansa/ts/built/local/tsserver.js:29663:28)
    at Object.getQuickInfoAtPosition (/home/nathansa/ts/built/local/tsserver.js:112086:34)
    at IOSession.Session.getQuickInfoWorker (/home/nathansa/ts/built/local/tsserver.js:119609:62)
    at Session.handlers.ts.createMapFromTemplate._a.(anonymous function) (/home/nathansa/ts/built/local/tsserver.js:118730:61)
    at /home/nathansa/ts/built/local/tsserver.js:120183:88
    at IOSession.Session.executeWithRequestId (/home/nathansa/ts/built/local/tsserver.js:120174:28)
    at IOSession.Session.executeCommand (/home/nathansa/ts/built/local/tsserver.js:120183:33)
    at IOSession.Session.onMessage (/home/nathansa/ts/built/local/tsserver.js:120203:35)
    at Interface.<anonymous> (/home/nathansa/ts/built/local/tsserver.js:121457:27)
    at emitOne (events.js:125:13)
    at Interface.emit (events.js:221:7)
    at Interface._onLine (readline.js:287:10)
    at Interface._normalWrite (readline.js:435:12)
    at Socket.ondata (readline.js:146:10)
    at emitOne (events.js:125:13)
    at Socket.emit (events.js:221:7)
    at addChunk (_stream_readable.js:265:12)
    at readableAddChunk (_stream_readable.js:252:11)
    at Socket.Readable.push (_stream_readable.js:209:10)
    at Pipe.onread (net.js:598:20)

Probably related? #26286

[sandersn]

Very likely to be a duplicate of, or at least related to, #26883 since it's a generic function that returns a class.

[sandersn]

Huh. This now works.