Over-aggressive pruning inside loops with try/catch

[jbt]

• Rollup Version: 1.14.0
• Operating System (or Browser): Any
• Node Version: 10.16.0

How Do We Reproduce?

Using the following code as input for rollup, no plugins and all the default options:

export default function mixin(Interface, Classes) {

    for (var _iterator2 = Object.getOwnPropertyNames(Interface)[Symbol.iterator](), _step2; !(_iteratorNormalCompletion2 = (_step2 = _iterator2.next()).done); _iteratorNormalCompletion2 = true) {
      var name = _step2.value;

      if (Class.hasOwnProperty(name)) continue;
      var desc = Object.getOwnPropertyDescriptor(Interface, name);
      Object.defineProperty(Class, name, desc);
    }

    try {
      for (var _iterator3 = Object.getOwnPropertyNames(Interface.prototype)[Symbol.iterator](), _step3; !(_iteratorNormalCompletion3 = (_step3 = _iterator3.next()).done); _iteratorNormalCompletion3 = true) {
        var _name = _step3.value;

        if (Class.prototype.hasOwnProperty(_name)) continue;
        var desc = Object.getOwnPropertyDescriptor(Interface.prototype, _name);
        Object.defineProperty(Class.prototype, _name, desc);
      }
    } catch (err) {
    }
}

(this is a reduced version of the mixin util from Slate.js after transpilation that can be found in lib/slate.es.js of slate on npm)

Expected Behavior

Output should be pretty much identical to input

Actual Behavior

The second loop gets reduced down to:

      for (; !(_iteratorNormalCompletion3 = (_step3 = _iterator3.next()).done); _iteratorNormalCompletion3 = true) {
        var desc;
      }

_iterator3 is no longer defined and the loop body is also missing.

This seems to be about as minimal as I can make the test case - removing the try/catch or anything inside either of the loops causes this behaviour to go away.

REPL link

This seems to have been introduced in 1.14.0, since with 1.13.1 it's perfectly fine (repl)

[lukastaegert]

Thanks, I will look into this, this seems strange indeed. Weirdly enough, the issue seems to arise from the variable desc that is defined inside the try-statement but accessed outside it. https://rollupjs.org/repl?version=1.14.0&shareable=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

[colinrobertbrooks]

I ran into this as well building this small repo.

[lukastaegert]

Thanks, I already tracked it down, fix upcoming.

[lukastaegert]

Fix at #2898