Compilation should fail when @silent does not suppress anything

[Daenyth]

Currently it seems like @silent can be added anywhere, even if it's not needed. This is particularly bothersome when it's applied to classes; a code change could fix the initial warning that's being ignored, but leave @silent in place, opening up accidental regression on further code changes.

[ghik]

That's an interesting idea. I don't think we can make what you propose the default behavior - it could break existing projects. However, we might hide it behind a flag.

[SethTisue]

fwiw, breaking existing projects seems okay to me here. the existing projects are likely to want to remove those stale annotations

[ghik]

@SethTisue what is the last compiler phase that can emit warnings?
Checking whether @silent has actually silenced something must happen after that phase.

[SethTisue]

even the jvm phase can emit warnings. (I don't call it the "last" phase because there's also terminal)

[ghik]

IMO, another reason to not enable this checking by default is that it may be problematic in cross-compiled projects where e.g. Scala 2.12 emits a warning but Scala 2.11 doesn't.

[ghik]

Fixed in 1.4.0