Add a failing test for nested fallbacks #2
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Fix encoding of Unicode keys greater than U+00FF
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May 23, 2024
…ferenced
identifiers
---
A few fixes for finding context identifiers:
Previously, we counted every babel identifier as a reference. This is
problematic because babel counts every string symbol as an identifier.
```js
print(x); // x is an identifier as expected
obj.x // x is.. also an identifier here
{x: 2} // x is also an identifier here
```
This PR adds a check for `isReferencedIdentifier`. Note that only non-lval
references pass this check
```js
print(x); // isReferencedIdentifier(x) -> true
obj.x // isReferencedIdentifier(x) -> false
{x: 2} // isReferencedIdentifier(x) -> false
x = 2 // isReferencedIdentifier(x) -> false
```
Which brings us to change #2.
Previously, we counted assignments as references due to the identifier visiting
+ checking logic. The logic was roughly the following (from facebook#1691)
```js
contextVars = intersection(reassigned, referencedByInnerFn);
```
Now that assignments (lvals) and references (rvals) are tracked separately, the
equivalent logic is this. Note that assignment to a context variable does not
need to be modeled as a read (`console.log(x = 5)` always will evaluates and
prints 5, regardless of the previous value of x).
```
contextVars = union(reassignedByInnerFn, intersection(reassigned,
referencedByInnerFn))
```
---
Note that variables that are never read do not need to be modeled as context
variables, but this is unlikely to be a common pattern.
```js
function fn() {
let x = 2;
const inner = () => {
x = 3;
}
}
```
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This adds a failing test for facebook#12279 which I discovered working on a movie demo.