[ts] highlighting breaks with a complex type guard

[mtaran-google]

const predicate = (values: [T, B]): values is
                    [T & {a: B},
                     B] => {
                    }

highlights fine, but if you press enter right before the second values

const predicate = (values: [T, B]): 
values is
                    [T & {a: B},
                     B] => {
                    }

does not

[marijnh]

The parser limits lookahead to two lines for performance reasons, and arrow function syntax is highly ambiguous until the arrow is seen—which with the extra newline here is more than two lines ahead from the first opening parentheses (the point where it has to decide what to parse).

We could look into loosening that, or move to a more fine-grained unit for the limit, like characters or tokens. Do you have any defined requirements or benchmarks for the highlighting performance?

[mtaran-google]

Interesting! We don't have any specific metrics for highlighting performance since it's not generally been a problem before. I've only hit this issue in a somewhat contrived coding situation (but still a real piece of code I needed to write). Maybe let's try bumping up the limit to 3 lines (maybe just for TS, since we've not had this reported for other langauges?) and I can circle back if we notice any performance issues coming out of it.

[marijnh]

Okay, if you install codemirror-grammar-mode 0.1.8, that version has a max lookahead of 3 lines.