Originally asked on reddit:
In Scala 2.13 I see this compiler option
-Wself-implicit Warn when an implicit resolves to an enclosing self-definition.
Why would I care about this? That is, what potential issue is it warning me about?
I tried cross compiling one of our projects at work and some things like scala.io.Source.fromBytes fail to compile because the implicit Codec instance was apparently a self-definition (in a supertrait of the Codec companion). Similarly ScalaTests using any of their implicit-based DSL fail to compile because many of those implicits are in mixin traits.
It seems like this warns about a very common practice for placing implicits to be found naturally and orderly, including in the standard library. So why would I ever use it? Is there a problem with such implicits or a principle that is violated?
Should I want to use the option eventually, is there a workaround where I can make such self-implicits be found in a way that doesn’t trigger the warning so I can still use libraries making use of the feature? Creating a new implicit value and referencing the value from the supertrait works but could get tedious quickly. Maybe the point is to have explicit local values/def/imports?