In JS, assignment to void 0 isn't a declaration

[sandersn]

Previously, property assignments with void 0 initialisers were treated like any other values. But this causes us to choke when checking our own commonjs emit. This is something that happens by mistake a fair amount, so this PR goes back to treating these assignments as normal assignments.

This should allow us to check our own emit in loose cases without harming other code bases, since void 0 is rarely written by hand.

Note that other errors still happen: noImplicitAny forbids accessing undeclared properties on object literals, and strictNullChecks forbids assigning undefined to properties with a different type. However, this change is enough to unblock compilation with strictNullChecks: false.

Fixes #38552

[sandersn]

@rachelgshaffer you might be interested in this as well.

[weswigham]

For the strict mode case, we'd need to do something like calculate the type via control flow to handle eliding the undefined assignment from the final type, right?

[sandersn]

@weswigham Control flow would be one way to do it. But we'd basically be lying about checking a type -- it would probably be easier to just skip the assignability check for lhs=export.* and rhs=void 0.

For the record, I don't think it's a good idea. This is basically an error-recovery feature, and I don't think disrupting assignability checks is worth it.