TypeScript 2.5.1 RC parse error (not in TS 2.4.2)

[JamesHenry]

As of v1.6 of Prettier we are using the TypeScript 2.5 RC and we have had a bug report for some code which worked fine pre-1.6 but is now broken.

Using tsc directly, I can confirm that when the following code is parsed with TypeScript 2.4.2 it produces an AST just fine, but when parsed with TypeScript 2.5.1 it produces "'=' expected."

Code

const foo = (x:string):void => (
  bar(
    x,
    () => {},
    () => {}
  )
);

Original issue: prettier/prettier#2702

[sandersn]

This is a result of parsing JSDoc types in normal typescript. JSDoc's function(number): number syntax has no parameter names, so now the start of a parameter can be the start of a type. But this results in nested lookahead when parsing arrow functions that fails in a way that I haven't fully understood.

There are two possible fixes: (1) fork the code paths (2) fix the lookahead in isStartOfType. Fix 1 is fast and simple but doesn't address the underlying issue: the lookahead in isStartOfType might not be necessary (or at least may be simplified).

Right now I'm planning to fix this bug in 2.5.3 and pursue option (2).

[sandersn]

Here's a smaller repro:

(bar(x,
    () => {},
    () => {}))

[JamesHenry]

Thanks, @sandersn!