Problems with optional chaining and nullish coalesce operators

[andreabergia]

While working on #1684 me and my colleagues @0xe and @nabacg have discovered a few problems with the two newly implemented operators.

For the optional chaining ?. we do not support the syntax a?.[ expr ], which #1684 would fix.
Also, while the original PR #1593 implemented partial support for optional function calling, i.e. a.b?.() works, it does not support properly other kind of function calls such as a?.() or a[b]?.().

For nullish coalesce operator ?? we have instead discovered that we "leak" some temporary state. In #1624 a fix was implemented so that something like f() ?? 42 would evaluate f() just once by creating a temporary name. However, that temp name leaks out to the containing scope, i.e.:

a = 1;
a ?? 42;
this.$0  // evaluates to "1"
Object.keys(this)  // will contain $0

Furthermore, we can even overwrite variables:

$0 = false;
a = true;
a ?? 42;
$0 // true

Given the similarities between these operators and the problems and missing features, we are trying to come up with a single approach that works with all cases. We hope to submit a PR in the next few weeks to fix all of this.

[andreabergia]

Should be fixed since the related PRs have been merged.