TypeScript Version: 3.2.0-dev.20180927
Search Terms: JSDoc conditional types

UsingConditional Types in JSDoc comments confuses the TypeScript parser since theT extends Y ? A : B syntax looks similar to theY? (meaningY|null) syntax from JSDoc.

Code

/** *@template {{}} T *@param {T} o *@returns {T extends Readonly<infer U> ? (keyof U)[] : string[]} */functionObject_keys(o){letkeys=Object.keys(o);//@ts-ignore: Type assertion for stripping `Readonly<…>`returnkeys;}

Expected behavior:

No error should be reported.
The type ofObject_keys should be:<T extends {}>(o: T): T extends Readonly<infer U> ? (keyof U)[] : string[].

Actual behavior:

TypeScript compiler shows an error ("?" expected) and mistakes the existing? operator for JSDoc<type>|null syntax.

The actual type therefor ends up being:<T extends {}>(o: T): T extends Readonly<infer U> | null ? (keyof U)[] : string[].

Possible fixes:

1. When encountering a top-level (not within parenthesis)? token in an extends clause, scan ahead and check whether it is followed by an| or& token (type intersection or union operators). If yes, treat it as|null and keep processing; otherwise, assume it's the start of a conditional type declaration. (Requires at least an LF(2) parser.)
2. Prohibit the JSDoc? operator in the top-level of an extends clause, always causing it to be treated as the start of the conditional type declaration.

Playground Link: None, Playground does not seem to support JavaScript with JSDoc instead of TypeScript as input.