GeezerVison

You can just pass parseNonParameterInitializer to lookahead. Testing truthiness is fine here, and you'll avoid allocating a closure.

I would not do it this way. I would just parse out an initializer for these properties. Then, in the checker, detect that and report a nice error there.

We do not want random errors being produced the parser like this.

The benefit of this is that now our tree is well formed. And you can do things like format the tree nicely and have the rest of our features not barf when this is encountered.

So to summarize Nathan Yee (@delta-nry), just add an optional initializer onto each PropertySignature and perform a grammar check in checker.ts on each property. Report the error on the initializer or the node itself.

I'm surprised that we don't go off the rails here since you're not advancing the parser at all (i.e. you use lookAhead, not tryParse).

Actually, this is because the parser tries to be smart and avoids reporting more errors at the same start position. I'm not sure if that's the right thing to do because instead of gracefully parsing the invalid initializer, the parser can start going haywire down the line. Maybe tryParse would be better? What do you think Cyrus Najmabadi (@CyrusNajmabadi)?

We should just allow initializers. This is a pretty reasonable thing for a user to type, so the tree shoudl just be flexible enough to represent it. We can hten have a simple grammar check in the type checker that reports that thsi isn't legal.

You should be performing this check in any type literal, not just those in variable like declarations.

I think you should just move the check into checkGrammarProperty, and discriminate on whether you have a PropertySignature with an initializer.

I'm not sure if we should accept these errors, as the '=' expected message implies that initializers are allowed in this case. Maybe we should spin this off into a new issue. Daniel Rosenwasser (@DanielRosenwasser) Cyrus Najmabadi (@CyrusNajmabadi), what are your thoughts on this?

It should be sufficient if you make it so that you only try to parse an initializer if the current token is a =.

Sorry I didn't notice it earlier, but this should be "A type literal property"