Incorrect handling of function default params when shadowed

[jjhuff]

Bug report

While we encountered this in our codebase, I can also repro with https://try.terser.org/

Version (complete output of terser -V or specific git commit)

$ yarn run terser -V
terser 5.25.0

Complete CLI command or minify() options used

// edit terser options
{
  module: true,
  compress: {},
  mangle: {},
  output: {},
  parse: {},
  rename: {},
}

terser input

export const exp = (t) => 1 ;

export function smoothDamp(easing = exp) {
  const exp = easing(10); 
}

terser output or error

export const exp=o=>1;export function smoothDamp(o=t){const t=o(10)}

Expected result

export const exp=o=>1;export function smoothDamp(o=exp){o(10)}

t in the output is undefined in global scope. In our case, it appears that it was getting assigned to another function -- leading to some really weird stuff. However, in the original case, it just doesn't work. By changing const exp to const e, the param default is handled correctly as exp.

Now, if you change const exp = easing(10); to be const e = easing(10); , you get correct output:

A coworker happened to try with esbuild:
https://esbuild.github.io/try/#dAAwLjE5LjgALS1taW5pZnkAZXhwb3J0IGNvbnN0IGV4cCA9ICh0KSA9PiAxIDsKCmV4cG9ydCBmdW5jdGlvbiBzbW9vdGhEYW1wKGVhc2luZyA9IGV4cCkgewogIGNvbnN0IGV4cCA9IGVhc2luZygxMCk7IAp9

[fabiosantoscode]

This is indeed a bug. And an incredibly hard one to fix. I think this is because there's a scope for the function's default parameters, and another for the function's variables. Thanks for reporting!

[jjhuff]

Cool, thanks! Yeah, function scopes are a pain.

[fabiosantoscode]

I think the advanced bits of this problem are impossible to fix such as the scope of var x here (from mishoo/UglifyJS#2662):

function(x, f = (() => x)) {
    var x;
    var y = x; // reading the argument `x`
    x = 2; // creating `var x`
    return [ x, y, f() ];
}(1);

This returns [2, 1, 1] because the inner x binding is only created by x = 2. Until then, accessing x goes to the outer scope, with the function args.