Incorrect transformation of nullish coalescing operator when passing "ecma" option.

[ruipin]

Bug report

Version (complete output of terser -V or specific git commit)
terser 5.6.1

Complete CLI command or minify() options used
terser test.js -c ecma=2020

Note that the issue can be reproduced with any value for ecma, e.g. terser test.js -c ecma=1, terser test.js -c ecma=2000000, terser test.js -c ecma, etc.

The issue does not occur if the ecma parameter is omitted, i.e. terser test.js.

terser input
A test.js file containing:

if(!(a ?? b))
    throw new Error("Some error");

terser output or error

if(a??!b)throw new Error("Some error");

Expected result

if(!(a??b))throw new Error("Some error");

For reference, this is the output when not passing any ecma parameter.

To see why the output is incorrect code, see e.g.

const a = 1;
const b = 2;
console.log(!(a ?? b)); // -> false
console.log(a ?? !b); // -> 1

[oxygene]

I'd like to note that this is quite a critical bug. It silently changes the logic of the application.

I have a Vue + webpack application with minification only active on production builds. So for use, this bug only becomes visible when the application hits QA, but the developers don't see it while coding.

[alexander-akait]

cc @fabiosantoscode I think critical