Use 'void' for the contextual type of an unused expression

[rbuckton]

This Draft PR primarily is intended to investigate the consequence of contextually typing unused expressions to void.

[rbuckton]

@typescript-bot run dt
@typescript-bot test this
@typescript-bot user test this

[typescript-bot]

Heya @rbuckton, I've started to run the extended test suite on this PR at b1d9034. You can monitor the build here.

[typescript-bot]

Heya @rbuckton, I've started to run the parallelized Definitely Typed test suite on this PR at b1d9034. You can monitor the build here.

[typescript-bot]

Heya @rbuckton, I've started to run the parallelized community code test suite on this PR at b1d9034. You can monitor the build here.

[rbuckton]

From what I can see, using void as the contextual type of any unused expression seems to be a major breaking change.

[rbuckton]

The most recent commit demonstrates only contextually typing unused await to void.

[rbuckton]

@typescript-bot run dt
@typescript-bot test this
@typescript-bot user test this

[typescript-bot]

Heya @rbuckton, I've started to run the parallelized community code test suite on this PR at 28b39d9. You can monitor the build here.

[typescript-bot]

Heya @rbuckton, I've started to run the parallelized Definitely Typed test suite on this PR at 28b39d9. You can monitor the build here.

[typescript-bot]

Heya @rbuckton, I've started to run the extended test suite on this PR at 28b39d9. You can monitor the build here.

[rbuckton]

@typescript-bot user test this

[typescript-bot]

Heya @rbuckton, I've started to run the parallelized community code test suite on this PR at 28b39d9. You can monitor the build here.

[typescript-bot]

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

[rbuckton]

From the user tests:

chrome-devtools-frontend:

node_modules/chrome-devtools-frontend/front_end/bindings/TempFile.js(77,9): error TS2322: Type '(value?: void | PromiseLike<void>) => void' is not assignable to type '(this: FileReader, ev: ProgressEvent<FileReader>) => any'.
  Types of parameters 'value' and 'ev' are incompatible.
    Type 'ProgressEvent<FileReader>' is not assignable to type 'void | PromiseLike<void>'.
      Property 'then' is missing in type 'ProgressEvent<FileReader>' but required in type 'PromiseLike<void>'.

Due to the fact the contextual type is void instead of unknown, this no longer becomes assignable to the onloadend handler of a FileReader. Addressing this would require either a cast of the promise or changing the code to reader.onloadend = () => resolve();

node_modules/chrome-devtools-frontend/front_end/ui/UIUtils.js(1995,65): error TS2345: Argument of type '(value?: void | PromiseLike<void>) => void' is not assignable to parameter of type '(arg0: Event) => any'.
  Types of parameters 'value' and 'arg0' are incompatible.
    Type 'Event' is not assignable to type 'void | PromiseLike<void>'.
      Property 'then' is missing in type 'Event' but required in type 'PromiseLike<void>'.

In this case, the resolve callback is no longer assignable to the click handler. Addressing this would require casting the promise.

puppeteer

src/node/BrowserFetcher.ts(335,64): error TS2345: Argument of type '(value?: void | PromiseLike<void>) => void' is not assignable to parameter of type '(error: Error) => void'.
  Types of parameters 'value' and 'error' are incompatible.
    Type 'Error' is not assignable to type 'void | PromiseLike<void>'.
      Property 'then' is missing in type 'Error' but required in type 'PromiseLike<void>'.

This is another case of resolve no longer being assignable to an existing callback.

[rbuckton]

At present this does not seem to be a tenable solution. From the user tests, there are several errors where contextually typing to void resulted in breaks in both JS and TS code, as void is too precise. As this was a change intended to reduce breaks caused by #39817, at this time it does not seem to be a viable solution.