Side effects function calls in parameters are dropped

[manucorporat]

We have never reproduced this issue on the wild, but I thought it could be a problem after going though the rollup's code. Low priority.

• Rollup Version: 1.14.6
• Operating System (or Browser): browser
• Node Version:

How Do We Reproduce?

https://rollupjs.org/repl?version=1.14.6&shareable=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

Expected Behavior

It should not drop the call to sideEffects(), since it might have side effects, it's not safe to drop.
Giving this entry-point:

function sideEffects() {
	console.log('print message');
	return true;
}

function hola(a, b) {
	console.log(a);
}

hola(1, sideEffects());

it should emit:

function sideEffects() {
	console.log('print message');
	return true;
}

function hola(a, b) {
	console.log(a);
}

hola(1, sideEffects());

Actual Behavior

It drops the side effect function if the parameter is unused.

function hola(a, b) {
	console.log(a);
}

hola(1);

so the console.log() is never printed

[lukastaegert]

Interesting, I distinctly remember having written code to prevent this. Will need to check again and add more tests.

[lukastaegert]

Thanks again for finding this, the fix was indeed as simple as an if instead of an else if. And a missed test case. Fix at #2924

[manucorporat]

Awesome job @lukastaegert ! you rock