Enhancement: Throw errors for invalid UpdateExpressions

[fisker]

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• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

Relevant Package

parser

My proposal is suitable for this project

• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

These expressions should be invalid.

1 ++;

(1)++;
(1)--;

++(1);
--(1);

(1 + 2)++;
(1 + 2)--;

++(1 + 2);
--(1 + 2);

(x + x)++;
(x + x)--;

++(x + x);
--(x + x);

[bradzacher]

These are all semantic errors in TS.
And looking at the eslint playground they're syntax errors in espree - so this seems like a good thing to error on.

[Josh-Cena]

Looking into it...

[Josh-Cena]

@bradzacher First time working with the parser. I have a few questions.

In #6589 (and other similar PRs), we use our own parser logic to generate the error. But we already have the allowlistSupportedDiagnostics function. Do these two reporting facilities serve different purposes?

Also, should I add test cases in ast-spec? How do I generate snapshots?

[bradzacher]

We avoid evaluating semantic diagnostics because they are slow to compute (~50ms per file). So instead we write our own logic to catch and report the errors.

The allowlisted errors thing isn't actually used in normal parsing. It's behind an option and I reckon we could probably remove it in a major.

Also, should I add test cases in ast-spec? How do I generate snapshots?

Yup - add test cases there - it's the only place we have them now!

This issue has a good rundown steps to add test cases (we should probably put this in the readme) #6065

Run them with cd packages/ast-spec && yarn test.