isReferenced is false for the left hand of AssignmentExpressions (revist #2557)

Bug Report

• [ x ] I’d be open to it if you think this is worth looking into

Current Behavior I apologize if this is already being discussed elsewhere, but I’d like to revisit bug #2557. If I can paraphrase that bug, after traversing the following code:

let x = 0;
let y = 0;

x += 1;
y++;

the x binding would not be considered referenced, while y would.

If I’m understanding the purpose of referenced correctly, I believe we would want these two behaviors (++ and +=) to be consistent with each other.

In the JS Packager Parcel, they use this reference counter in their scope hoisting functionality, which causes incorrect tree shaking behavior for variables in += AssigmentExpressions. Bug.

[mischnic]

There is some logic behind this:

// causes binding.constant to be false, is a constantViolation, not a referencePath
x = 2; 

// causes binding.constant to be false, is a constantViolation, not a referencePath
x += 2;

// causes binding.referenced to be true and constant to be false,
// is a referencePath **and** a constantViolation
x++; 

// causes binding.referenced to be true, is a referencePath, not a constantViolation
console.log(x);

Would be great to get some feedback, but I guess this is per design and we’ll fix it in Parcel.

[nicolo-ribaudo]

We have “references” that are “reads”, and “constantViolations” that are “writes”.

Actually, I’m curious how does Babel even rewrite that correctly?

Maybe you are calling replaceWith on both identifiers?