This PR now uses the grammar check functionality in the checker rather than the parser as we discussed in the design meeting, however, I don't think that this is working how I'd desire.

grammarErrorOnNode only reports the error if the file has no other parser errors, so a number of our tests don't actually error, while they do error for other illegal constructs like a<b>[...]. See: 728c9b4 (#49464)

I made the change I prefer in #49645 to show the difference. See: 2925fcf (#49645)

Can you remind me why we decided to make this a grammar check?

This was the suggestion by @ahejlsberg, as a potentially better recovery method. But, I think #49645's method here is better; the tree is the same in both cases, but it correctly fires when it's done in the parser (whereas grammar checks do not always fire).

Updated to be in the parser, and to move the error message onto the type parameters.

@typescript-bot cherry-pick this to release-4.7

Heya @jakebailey, I've started to run the task to cherry-pick this into release-4.7 on this PR at db3e062. You can monitor the build here.

Hey @jakebailey, I couldn't open a PR with the cherry-pick. (You can check the log here). You may need to squash and pick this PR into release-4.7 manually.