fixups

athema/src/main/scala/higherkindness/athema/package.scala

@@ -4,18 +4,20 @@ import cats.Functor
 import higherkindness.droste._
 import higherkindness.droste.data.Fix
 
+import scala.annotation.nowarn
+
 object `package` {
 
   implicit val functorExprFixed: Functor[Expr.Fixed] = new Functor[Expr.Fixed] {
     def map[A, B](fa: Expr.Fixed[A])(f: A => B): Expr.Fixed[B] =
       scheme.cata(mapAlgebra(f)).apply(fa)
   }
 
+  @nowarn("msg=match may not be exhaustive")
   private def mapAlgebra[A, B](f: A => B): Algebra[Expr[A, *], Expr.Fixed[B]] =
     Algebra {
       case Const(c)                        => Fix(Const(f(c)))
       case other: Expr.Fixed[B] @unchecked => other
-      case _                               => sys.error("impossible to reach")
     }
 
 }

build.sbt

@@ -32,7 +32,6 @@ lazy val coverage = (project in file(".coverage"))
     noPublishSettings,
     crossScalaVersions := Seq(ScalaV.v212, ScalaV.v213),
     scalaVersion := ScalaV.v213
-//    scalacOptions ++= on(2, 13)("-Ytasty-reader").value
   )
   .settings(coverageEnabled := true)
   .aggregate(coreJVM)
@@ -204,7 +203,9 @@ lazy val athema = module("athema", prefix = "")
       ) ++
         Seq(
           "org.scalacheck" %%% "scalacheck" % V.scalacheck
-        ).map(_ % "test")
+        ).map(_ % "test"),
+    // 'nowarn' doesn't work on scala3 yet, make warnings not fatal.
+    scalacOptions --= on(3)("-Xfatal-warnings").value
   )
 
 lazy val athemaJVM = athema.jvm

modules/core/src/main/scala/higherkindness/droste/basis.scala

@@ -43,7 +43,6 @@ trait Project[F[_], R] { self =>
     Project.collect[F, R, U, B](r)(pf)
 
   def contains(r: R, c: R)(implicit R: Eq[R], F: Foldable[F]): Boolean =
-    // For some reason, this is ambiguous in scala3 unless I declare it here.
     Project.contains[F, R](r, c)(tc, R, F)
 
   def foldMap[Z: Monoid](r: R)(f: R => Z)(implicit F: Foldable[F]): Z =