Tree-shaking drop useful code

[straccia17]

Tree-shaking algorithm seems to be too aggressive; it drops some useful code breaking runtime feature detection. The dropped code is part of a module of core-js (v2.6.7) installed as dependencies of @babel/polyfill (v.7.4.4).

• Rollup Version: 1.12.3
• Operating System (or Browser): macOS Sierra 10.13.6
• Node Version: 8.11.1

How Do We Reproduce?

Clone this repository https://github.com/straccia17/rollup-treeshake-bug, execute yarn, then yarn run build.

Expected Behavior

Expected behavior is shown by dist/bundle-fixed.js files

Actual Behavior

Actual behavior is shown by dist/bundle.js files

Workaround

Set treeshake: false in rollup.config.js

[lukastaegert]

Duplicates/related: #2540, #2273, #1771

The reason the code is removed is that Rollup always assumes that the code is running in a current generation environment when deciding if the call to a global function/contstructor has side-effects (such as throwing an error). This of course is a big problem for feature detection and finding solutions for this is definitely on my to-do list.

Even though there are already issues for this topic, I really like your reproduction as it shows nicely the code that is generated by core-js and we can readily use it to write tests.

Even though I stated otherwise in #2273, I think now that deoptimizing try-catch statements with regard to tree-shaking might actually be a good compromise way forward if we manage to track situations as in your example.

Alternative approaches are:

• provide a user option to list or determine via a function files which should not be tree-shaken
• open up the API to define side-effects for globals (more useful in marking stuff as side-effects-free IMO)
• a special (comment) syntax to retain code (would rely on adoption by library authors, though. We might consider working with core-js if we want this).

[lukastaegert]

#2892 will provide a solution for this that should work for you, comments welcome.