JSDoc type assertions for expression without parens

[mjbvz]

From #5158

TypeScript Version: 2.5.2

Code
For the JavaScript:

//@ts-check

const img = /** @type {HTMLImageElement} */ document.getElementById('#cat')
img.src = 'cat.gif'

Expected behavior:
The type of img is HTMLImageElement

Actual behavior:
The type of img is any. You currently need parens around the expression for this to work properly:

//@ts-check

const img = /** @type {HTMLImageElement} */ (document.getElementById('#cat'))
img.src = 'cat.gif'

I feel that the parens should not be required. The JSDoc cast should have the lowest precedence and apply to the entire expression on the right hand side

[mhegazy]

this is the intended design. looking up these cast expressions in comments is not cheep. moreover, scoping rules are not clear either.
we made a design choice to only look them up for parenthesized expressions.

[weswigham]

The bigger issue, I think, is that we'd need to start parsing and maintaining JSDoc ASTs on all expressions - and we're very frugal about where we do that right now. The actual lookup is fairly cheap - no moreso than a normal TS-style cast, really (only a slightly more complicated AST to traverse to find the type to cast to).

[mjbvz]

Ok, if there is a technical reason why the parens necessary, feel free to close this out. I only opened this because when I first tried to write a jsdoc cast I forgot the parens around the expression, and it took me a moment to realize this