Add basic defaults for mimaPreviousArtifacts

Kind of like sbt-mima-version-check

src/main/scala/sbtcompatibility/SbtCompatibilityMima.scala

@@ -0,0 +1,20 @@
+package sbtcompatibility
+
+import com.typesafe.tools.mima.plugin.MimaPlugin
+import sbt.{AutoPlugin, Def, Keys}
+import sbtcompatibility.version.Version
+
+object SbtCompatibilityMima extends AutoPlugin {
+
+  override def trigger = allRequirements
+  override def requires = MimaPlugin
+
+  override def projectSettings = Def.settings(
+    MimaPlugin.autoImport.mimaPreviousArtifacts := {
+      val projId = Keys.projectID.value
+      val versions = Version.latestCompatibleWith(sbt.Keys.version.value).toSet
+      versions.map(version => projId.withRevision(version))
+    }
+  )
+
+}

src/sbt-test/sbt-compatibility/simple/build.sbt

@@ -44,6 +44,7 @@ lazy val d = project
     ),
     compatibilityDefaultReconciliation := lmcoursier.definitions.Reconciliation.Strict,
     checkFails,
+    checkMimaPreviousArtifactsSet,
     version := "0.1.1"
   )
 
@@ -52,17 +53,28 @@ inThisBuild(List(
   organization := "io.github.alexarchambault.sbtcompatibility.test",
 ))
 
+lazy val check = taskKey[Unit]("")
+
 lazy val shared = Def.settings(
-  compatibilityPreviousArtifacts := compatibilityAutoPreviousArtifacts.value
+  compatibilityPreviousArtifacts := compatibilityAutoPreviousArtifacts.value,
+  check := {}
 )
 
-lazy val check = taskKey[Unit]("")
-
 lazy val checkFails = Def.settings(
   check := {
+    check.value
     val direction = compatibilityCheckDirection.value
     val reports = compatibilityFindDependencyIssues.value
     val failed = reports.exists(!_._2.validated(direction))
     assert(failed, s"Expected a failed report in $reports")
   }
 )
+
+lazy val checkMimaPreviousArtifactsSet = Def.settings(
+  check := {
+    check.value
+    val previousArtifacts = mimaPreviousArtifacts.value
+    val versions = previousArtifacts.map(_.revision)
+    assert(versions.nonEmpty, "No MiMa previous artifact found")
+  }
+)