Function param override gets incorrectly inlined

[dgreif]

Describe the bug

SWC compress seems to be inlining values for a param override, even when there are paths where the override might not happen. This does not happen for local variables (e.g. let output = 'a output'), only for variables which come from function params.

Input code

function run(flag: 'a' | 'b', output = 'a output') {
  if (flag === 'b') {
    output = 'b output'
  }

  console.log(output)
}

run('a')
run('b')

Config

No response

Playground link (or link to the minimal reproduction)

https://play.swc.rs/?version=1.3.79&code=H4sIAAAAAAAAA02NUQqAIBBE%2Fz3F%2FK1CdIDAw2hkBOJG6Vd199Ys6GcZ3szshJLGvHDCVpIO0c0DyBFOkKcOXPJaMqywV5PBoYAl4EnDWlujjeJX8F9B%2BKXkjJx2jlMfedbNMkqMuiuLpgn5pG7xOwn0kwAAAA%3D%3D&config=H4sIAAAAAAAAA4VWO2%2FbMBDe8ysCzS4cZOjQqWiDoF26dCwKgSaP8iV8COQxsRDkv%2FdIKZZtUe5m8%2Ft4j%2B8e1NvN7W3zFGXz5faNf%2FIfOBAEJ8wPMD2EyIAWJsKmoIz3IkQIR345i4MjceCzhoYeogzYU7OZcYoZpJDg5FANTliUP23vA1VgkD4I8iWEjE3Q%2B0kktI%2FngQQQkj4pb7d98BoNuo4Jf44EpmxffXiOvZAQtx3SPu22zitorVfJ8NnXksL2aKk5Xv67iOAZoP9uRIy%2FhIV4kUNDInSQM2sg3t%2Fd3zUzEoSL2gd7Hn2K8AAaHTz6UMw%2BIhi1KAEzDXRCDg8fEi3VK%2BGfWedDLFL%2F9ilIyGGNpM0pJSRHaAsqEnkrCOU5g4P8ltAQuovCsDALgSw61MN5ltLbPkCMl9GxWsmCo0q%2BBQ3%2B9VLiguy8NyDcVawVsUVH0FU6egrKGNFHaF9EqFrKUYuA0dcdZTgRqJb7rl9hOIWEnkeriisQqpXciVUQA0jCl0WTjSj75KsucrLV7ApBwS51HSybJQ%2F9izBJUNU3HEq5OPKq7b3HSK1Ori7sCK%2BqMsKT6MvbqNsAlIKr3X3y6FbrlUezlXmI3DSbS%2BuFo7nv1i3o%2F9xGx%2FOKNFQZxq9k7Xh8CVpEXVU8qwWBsF7tACpJyIrLemATYVXUiLzuQGvuqKqD%2BIok93XneTl6XYW4A4Su998ItccZXmXkMbpKeOSsaa0VJw4vrf01PA52581VNxZo72ubd6ZwkchfIwTeNof%2BGiM5Vda9WiHxqlUrq4SHhnxrYPlwlv7hoWK7bWf8bl43854%2BeSiscN28NT4W%2BM1EasZXcf46yPUfX7TPzUz62PKjjfd%2FTAene1UIAAA%3D

SWC Info output

No response

Expected behavior

In the code above, flag can be either a or b. I would expect the flag === 'b' check to remain in the output:

function run(flag, output = 'a output') {
    'b' === flag && (output = 'b output'), console.log(output);
}
run('a'), run('b');

Actual behavior

With SWC compress turned on, it appears to assume that flag === 'b' is always true and inlines the output override value.

function run(flag, output = 'a output') {
    console.log('b output');
}
run('a'), run('b');

Version

1.3.79+

Additional context

In the playground I was able to confirm correct behavior in 1.3.78, with 1.3.79+ having issues. I looked through the changelog for 1.3.79 but nothing immediately jumped out to me as a definite cause. Maybe #7839?

[swc-bot]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.