Cool, thanks for reporting!

Reduced example

Complete CLI command or minify() options used

{
  module: true,
}

terser input

function causeSideEffect(a = console.log('sideEffect')) {}
causeSideEffect();

terser output or error

<nothing>

Expected result

console.log('sideEffect')

Appreciate it!

I'm working on this currently, it's going OK, but don't expect too much as I'm not very familiar with the code base.

It seems pretty hard to me, after writing a test and trolling through the code. Side effects in default assignments have to be treated specially, but there's lots of places where arguments are just assumed (without calling any methods to find out) to have no side effects.