`open object` is allowed despite not doing anything

[KacperFKorban]

Compiler version

3.6.1-RC1-bin-20241011-5e7ab15-NIGHTLY

Minimized code

//> using scala 3.nightly
  
open object O // no effect

Output

Successful compilation.

Expectation

Warning or error that open objects trivially cannot be extended.

Other info

This check should most likely be done in Desugar.scala#checkModifiers or somewhere in Checking.scala (maybe checkWellFormed)

[mbovel]

This issue was picked for the Scala Issue Spree of Monday, November 11th. @SethTisue and @HarrisL2 will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

[SethTisue]

sealed object errors in both Scala 2 and 3, so that seems like precedent that we should error on this too, since the combination is equally nonsensical (rather than merely redundant)