When the compression is enabled, used variables are dropped.

[Sayan751]

Bug report

As the title already says, when the compression is enabled, even some used variables get dropped. Sometimes it is also happens for imported modules when static blocks come into picture.

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

Complete CLI command or minify() options used

NA

terser input

const testVarOne = 'Hello this is test var one';

const testVarTwo = (/* unused pure expression or super */ null && (`
${testVarOne}

Hello this is test var two
`));

const testObj = {
  one: `${testVarOne}`
};

let _initClass;
function fn($class) {
  return [class extends $class {}, function () {}];
}

let _MyElement;
class MyElement {
  static {
    [_MyElement, _initClass] = fn(this);
  }
  prop = testObj.one;
  static {
    _initClass();
  }
}

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

terser output or error

const t=`${testVarOne}`;let s,a;class c{static{var t;[a,s]=(t=this,[class extends t{},function(){}])}prop=t;static{s()}}

Expected result

const t="Hello this is test var one";let s,a;class c{static{var t;[a,s]=(t=this,[class extends t{},function(){}])}prop=t;static{s()}}

[liuxingbaoyu]

This doesn't seem to reproduce the problem because you forgot $ before {testVarOne}.
babel/babel#16453 (comment) may be a reproducible code.

[Sayan751]

This doesn't seem to reproduce the problem because you forgot $ before {testVarOne}. babel/babel#16453 (comment) may be a reproducible code.

Thanks @liuxingbaoyu for pointing it out. Updated the original example.

[fabiosantoscode]

I managed this reproduction with less elements

const testVarOne = 'Please preserve this';

const usedOnce = testVarOne
const usedTwice = testVarOne

class MyElement {
  static {
    // class refers to itself AND has side-effects.
    sideEffect(this);
  }
  notDeletedReference = usedTwice;
}

The class MyElement looks like it will be dropped because it's unused. But it's not dropped because sideEffect(this) is a use of the class. Using sideEffect(MyElement) would fix the issue, which tells me the problem probably showed up when Terser started to recognize a class this as a reference to it.