Incorrect tree-shaking with switch + default

[jbt]

• Rollup Version: 1.25
• Operating System (or Browser):
• Node Version:

How Do We Reproduce?

The following code gets aggressively tree-shaken and all the functional parts get removed:

var bar;
switch (foo) {
	default:
		bar = 1;
}

console.log(bar)

REPL version

Expected Behavior

Output code should be pretty much the same as input code

Actual Behavior

Pretty much everything important gets accidentally removed:

'use strict';

switch (foo) {
	}

This is a reduced case of a bug that's causing ReactDOM to break in 1.25 (it's fine with 1.23, and was broken in 1.24 due to #3168) - the functional parts of commitAttachRef get removed: Reduced REPL version. It looks like 1.24 actually treats this behaviour fine - REPL - so perhaps this bug arose out of the fix for #3168.

[lukastaegert]

Thanks a lot for spotting this. The issue was a simple oversight with weird effects that only showed in very particular situations. To be precise, the switch statement is included due to a side-effect in the condition (in this case because it is an unknown global) at a point where the cases themselves have no side-effect (because the console.log is not yet included) resulting in no code after the switch statement being included, preventing the side-effect (in fact, the statement was treated like a throw). Fix at #3178