Attempt to make this work with scala/scala3#3421

Replace the use of ambiguous implicits by the `scala.implicits.Not`
class introduced in the PR. Running `sbt dottyThing/compile` crashes
with:
[info] exception occurred while compiling /home/smarter/opt/shims/dottyThing/src/main/scala/shims/dotty/test.scala
java.lang.AssertionError: assertion failed
        at dotty.DottyPredef$.assertFail(DottyPredef.scala:38)
        at dotty.tools.dotc.core.TyperState.commit(TyperState.scala:135)
        at dotty.tools.dotc.typer.Implicits.op$12(Implicits.scala:737)
        at dotty.tools.dotc.typer.Implicits.op$2(Implicits.scala:732)
        at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:728)
        at dotty.tools.dotc.typer.Implicits.inferImplicitArg(Implicits.scala:615)
        at dotty.tools.dotc.typer.Typer.implicitArgs$2(Typer.scala:2020)
        at dotty.tools.dotc.typer.Typer.implicitArgs$2(Typer.scala:2030)
        at dotty.tools.dotc.typer.Typer.addImplicitArgs$1(Typer.scala:2033)
        at dotty.tools.dotc.typer.Typer.adaptNoArgsImplicitMethod$2(Typer.scala:2085)
        at dotty.tools.dotc.typer.Typer.adaptNoArgs$1(Typer.scala:2201)
        at dotty.tools.dotc.typer.Typer.adaptInterpolated(Typer.scala:2346)
        at dotty.tools.dotc.typer.Typer.adaptInterpolated(Typer.scala:2335)
        at dotty.tools.dotc.typer.Typer.op$82(Typer.scala:1872)
        at dotty.tools.dotc.typer.Typer.op$42(Typer.scala:1868)
        at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:1867)
        at dotty.tools.dotc.typer.Typer.op$40(Typer.scala:1706)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1702)
....

Author: smarter

build.sbt

@@ -157,6 +157,7 @@ lazy val core = crossProject
       "org.typelevel" %%% "cats-core"   % CatsVersion,
       "org.typelevel" %%% "cats-free"   % CatsVersion,
       "org.scalaz"    %%% "scalaz-core" % ScalazVersion,
+      "ch.epfl.lamp"  %%  "dotty-library" % "0.5.0-bin-SNAPSHOT-nonbootstrapped",
 
       "com.chuusai"   %%% "shapeless"   % "2.3.2",
 
@@ -169,6 +170,10 @@ lazy val coreJS = core.js
 
 lazy val dottyThing = project
   .dependsOn(coreJVM)
+  .settings(
+    projectDependencies ~= (_.map(_.withDottyCompat())),
+    scalaVersion := "0.5.0-bin-SNAPSHOT"
+  )
 
 enablePlugins(GitVersioning)
 

core/src/main/scala/shims/util/nsub.scala renamed to core/src/main/scala/shims/util/package.scala

@@ -14,18 +14,8 @@
  * limitations under the License.
  */
 
-package shims.util
+package shims
 
-import scala.annotation.implicitNotFound
-
-@implicitNotFound("unable to prove that $A is NOT a subtype of $B (likely because it actually is)")
-sealed trait </<[A, B]
-
-private[util] trait NSubLowPriorityImplicits {
-  implicit def universal[A, B]: A </< B = null
-}
-
-object </< extends NSubLowPriorityImplicits {
-  implicit def lie1[A, B](implicit ev: A <:< B): A </< B = null
-  implicit def lie2[A, B](implicit ev: A <:< B): A </< B = null
+package object util {
+  type </<[A, B] = scala.implicits.Not[A <:< B]
 }

project/plugins.sbt

@@ -24,3 +24,4 @@ addSbtPlugin("com.jsuereth"   % "sbt-pgp"      % "1.0.0")
 addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "0.5.0")
 
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.14")
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.1.6")