InferenceMatchablePlugin.scala

import dotty.tools.dotc.ast.tpd.TypeTree
import dotty.tools.dotc.core.Contexts.Context
import dotty.tools.dotc.core.Symbols.defn
import dotty.tools.dotc.plugins.{PluginPhase, StandardPlugin}
import dotty.tools.dotc.typer.FrontEnd
import dotty.tools.dotc.report

class InferenceMatchablePlugin extends StandardPlugin {
  override def name = "inference-matchable-plugin"

  override def description = "warn Matchable"

  override def init(options: List[String]): List[PluginPhase] = {
    (new InferenceMatchablePhase()) :: Nil
  }
}

class InferenceMatchablePhase extends PluginPhase {
  override def phaseName = "inference-matchable"

  override val runsAfter = Set(FrontEnd.name)

  override def prepareForTypeTree(tree: TypeTree)(using context: Context): Context = {
    if (tree.tpe == defn.MatchableType) {
      report.warning("Matchableに推論されてるぞ！！！", tree.srcPos)
    }
    context
  }
}