Dead code detection too aggressive on array prototype mutation

[maximilianMairinger]

Rollup Version

2.69.0

Operating System (or Browser)

MacOs chrome

Node Version (if applicable)

No response

Link To Reproduction

https://rollupjs.org/repl/?version=2.69.0&shareable=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

Expected Behaviour

Executing the bundled output should yield the same result as executing the source.

The dead code detection should assume the same possibility for side effects in the prototype for natively declared array instances as it does for those declared with new Array().

Actual Behaviour

I assume it assumes we are accessing a property that doesn't exist (as it usually doesn't on arrays), which is very ambitious, but not save as illustrated in the repl above.

[lukastaegert]

Yes, this is a known limitation that we assume built-ins have not been tampered with. One could argue, though, that with respect to future prototype features that are not yet known to rollup, missing prototype properties should be handled as unknown instead of undefined.

[lukastaegert]

I created a PR to improve this with respect to handling future prototype methods gracefully: #4428

[maximilianMairinger]

Looks great thanks!