Fixes contains bug

Ref https://github.com/scala/bug/issues/10831
Ref https://github.com/sbt/sbt/pull/4139#commitcomment-29633458

main/src/main/scala/sbt/internal/CrossJava.scala

@@ -161,7 +161,8 @@ private[sbt] object CrossJava {
           case Some(v) =>
             projectJavaVersions flatMap {
               case (proj, versions) =>
-                if (versions.isEmpty || versions.contains(v)) Vector(proj -> versions)
+                if (versions.isEmpty || versions.contains[String](v.toString))
+                  Vector(proj -> versions)
                 else Vector()
             }
         }
@@ -184,7 +185,7 @@ private[sbt] object CrossJava {
 
       val filterKeys: Set[AttributeKey[_]] = Set(javaHome).map(_.key)
 
-      val projectsContains: Reference => Boolean = projects.map(_._1).toSet.contains
+      val projectsContains: Reference => Boolean = projects.map(_._1).toSet[Reference].contains(_)
 
       // Filter out any old javaHome version settings that were added, this is just for hygiene.
       val filteredRawAppend = session.rawAppend.filter(_.key match {

sbt/src/sbt-test/java/cross/changes/build.sbt

@@ -0,0 +1,25 @@
+import complete.DefaultParsers._
+
+val check = inputKey[Unit]("Runs the check")
+
+lazy val root = (project in file("."))
+  .settings(
+    ThisBuild / scalaVersion := "2.12.6",
+    crossJavaVersions := List("1.8", "10"),
+
+    // read out.txt and see if it starts with the passed in number
+    check := {
+      val arg1: Int = (Space ~> NatBasic).parsed
+      file("out.txt") match {
+        case out if out.exists =>
+          IO.readLines(out).headOption match {
+            case Some(v) if v startsWith arg1.toString => ()
+            case Some(v) if v startsWith s"1.$arg1"    => ()
+            case x => sys.error(s"unexpected value: $x")
+          }
+        case out => sys.error(s"$out doesn't exist")
+      }
+    },
+
+    Compile / run / fork := true,
+  )

sbt/src/sbt-test/java/cross/test

@@ -4,3 +4,12 @@
 > java++ 10!
 > run
 > check 10
+
+# test safe switching
+$ copy-file changes/build.sbt build.sbt
+$ delete out.txt
+> reload
+> clean
+> java++ 10
+> run
+> check 10