Incorrect null check after await

[rebornix]

After await call, the _editorModelReference is possible to be null and the type should still be IReference<IResolvedNotebookEditorModel> | null other than never.

[usernamehw]

Maybe a typo: this._editorModelReference was used instead of editorModelReference?

this._editorModelReference was checked in the if above that it's falsy, then it was checked again if it's truthy without changing the variable value. never seems to be the right type in that case.

[rebornix]

@usernamehw this function can be called multiple times, and the async task resolve takes a long time to finish, so for all calls the first this._editorModelReference is null and then stop and wait for the promise the resolve. When the promise resolves, the first call continues, and it sets this._editorModelReference to an object; for the second calls, the this._editorModelReference is no longer null and they will override this._editorModelReference.

[yume-chan]

It's a common pitfall of TypeScript's control flow analysis, see microsoft/TypeScript#9998 (it's opened in 2016!).

TypeScript can't dig into every function to check whether they change any field (due to performance reason), so it assumes all function to have no side effects.

[rebornix]

@yume-chan thanks for the pointer, yes it's definitely a limitation of TypeScript.

[mjbvz]

Closing as upstream