Disallow bad comma left operands

[RyanCavanaugh]

Fixes #10814

This PR disallows the left operand of the comma operator to be an expression form whose evaluation is known to not have side effects. The allowUnreachableCode option suppresses this warning, as does the unary void operator.

The check for "might have side effects" is shallow, which might be considered surprising. The intuition here is that

let y = 0;
let x = ([1, 2, y++], 4);

is not made valid by the side effect inside the unused array literal.

[mhegazy]

baselines needs updating.

[mhegazy]

what about the condition part? it could have side effects.

[RyanCavanaugh]

It could, but that's not germane -- [((x++ ? 3 : 4), 7), 3] is still not a plausibly-correct expression. If both the true and false expressions don't have side effects, there's no reason to be using the ? operator

[mhegazy]

👍

[RyanCavanaugh]

🐌 Travis 🐌

[vladima]

👍