No warnings can be incurred under -Xfatal-warnings

I am trying to import packages into sbt console as the following:

scala> import cats.instances.string
<console>:11: warning: Unused import
       import cats.instances.string
                             ^
error: No warnings can be incurred under -Xfatal-warnings.

and you can see, I've got an error message.

The content of the build.sbt is:

scalaVersion := "2.12.8"

scalacOptions ++= Seq(
  "-encoding", "UTF-8",   // source files are in UTF-8
  "-deprecation",         // warn about use of deprecated APIs
  "-unchecked",           // warn about unchecked type parameters
  "-feature",             // warn about misused language features
  "-language:higherKinds",// allow higher kinded types without `import scala.language.higherKinds`
  "-Xlint",               // enable handy linter warnings
  "-Xfatal-warnings",     // turn compiler warnings into errors
  "-Ypartial-unification" // allow the compiler to unify type constructors of different arities
)

libraryDependencies += "org.typelevel" %% "cats-core" % "1.4.0"
libraryDependencies += "org.tpolecat" %% "atto-core"    % "0.6.5"
libraryDependencies += "org.tpolecat" %% "atto-refined" % "0.6.5"

addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.9.3")

What am I doing wrong?

Solution

The best solution in this situation is to remove -Xlint from the Scala options that are used for the console:

scalaVersion := "2.12.8"
scalacOptions ++= Seq(
  "-Xlint",
  "-Xfatal-warnings"
)

scalacOptions in (Compile, console) ~= {
  _.filterNot(Set("-Xlint"))
}

libraryDependencies += "org.typelevel" %% "cats-core" % "1.6.0"

With this configuration, any source code in your project will be compiled with -Xlint, but any code that's interpreted in the REPL won't be. This is generally exactly what you want: the most thorough safety-checking possible for your project code, but much more flexibility for experimentation in the REPL.