Add plugin cross-build override test

Cover an AutoPlugin that sets pluginCrossBuild / sbtVersion
and a concrete sbt plugin that requires both that AutoPlugin and
SbtPlugin.

Also assert that a plain Scala 2.12 SbtPlugin project keeps the
existing 1.5.8 default, and that an explicit value equal to the
running sbt version is preserved.

sbt-app/src/sbt-test/plugins/plugin-cross-sbt-version-override/build.sbt

@@ -0,0 +1,38 @@
+lazy val check = taskKey[Unit]("Checks pluginCrossBuild / sbtVersion")
+
+lazy val root = (project in file("."))
+  .enablePlugins(ConcreteSbtPlugin)
+  .settings(
+    scalaVersion := "2.12.21",
+    check := {
+      val actual = (pluginCrossBuild / sbtVersion).value
+      val expected = "1.12.12"
+      assert(actual == expected, s"Expected pluginCrossBuild / sbtVersion to be $expected, got $actual")
+
+      val defaultActual = (defaultPlugin / pluginCrossBuild / sbtVersion).value
+      val defaultExpected = "1.5.8"
+      assert(
+        defaultActual == defaultExpected,
+        s"Expected default pluginCrossBuild / sbtVersion to be $defaultExpected, got $defaultActual"
+      )
+
+      val sameAsRunningActual = (sameAsRunning / pluginCrossBuild / sbtVersion).value
+      val sameAsRunningExpected = sbtVersion.value
+      assert(
+        sameAsRunningActual == sameAsRunningExpected,
+        s"Expected explicit pluginCrossBuild / sbtVersion to be $sameAsRunningExpected, got $sameAsRunningActual"
+      )
+    }
+  )
+
+lazy val defaultPlugin = (project in file("default-plugin"))
+  .enablePlugins(SbtPlugin)
+  .settings(
+    scalaVersion := "2.12.21"
+  )
+
+lazy val sameAsRunning = (project in file("same-as-running"))
+  .enablePlugins(SameAsRunningPlugin)
+  .settings(
+    scalaVersion := "2.12.21"
+  )

sbt-app/src/sbt-test/plugins/plugin-cross-sbt-version-override/project/PluginCrossSbtVersionOverridePlugin.scala

@@ -0,0 +1,25 @@
+import sbt._
+import sbt.Keys._
+
+object PluginCrossSbtVersionOverridePlugin extends AutoPlugin {
+  override def projectSettings: Seq[Def.Setting[?]] = Seq(
+    pluginCrossBuild / sbtVersion := {
+      scalaBinaryVersion.value match {
+        case "2.12" => "1.12.12"
+        case _      => sbtVersion.value
+      }
+    }
+  )
+}
+
+object ConcreteSbtPlugin extends AutoPlugin {
+  override def requires = PluginCrossSbtVersionOverridePlugin && plugins.SbtPlugin
+}
+
+object SameAsRunningPlugin extends AutoPlugin {
+  override def requires = plugins.SbtPlugin
+
+  override def projectSettings: Seq[Def.Setting[?]] = Seq(
+    pluginCrossBuild / sbtVersion := sbtVersion.value
+  )
+}

sbt-app/src/sbt-test/plugins/plugin-cross-sbt-version-override/test

@@ -0,0 +1 @@
+> check