[2.x] fix: Fixes java++ tab completion (#8778)

Fixes #4310

Per review, the OOM comes from the version parser - token(StringBasic)
on the right side of || has no examples constraint. Move
.examples(knownVersions*) from the inner number parser to wrap the
entire || expression, so both alternatives are bounded.

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

@@ -132,6 +132,9 @@ private[sbt] object CrossJava {
   private case class SwitchTarget(version: Option[JavaVersion], home: Option[File], force: Boolean)
   private case class SwitchJavaHome(target: SwitchTarget, verbose: Boolean, command: Option[String])
 
+  private[sbt] val JavaSwitchCommandCompletions =
+    Vector("-v", "compile", "test", "run", "clean", "console", "package")
+
   private def switchParser(state: State): Parser[SwitchJavaHome] = {
     import DefaultParsers.*
     def versionAndCommand(spacePresent: Boolean) = {
@@ -141,9 +144,9 @@ private[sbt] object CrossJava {
       val knownVersions = javaHomes.keysIterator.map(_.numberStr).toVector
       val version: Parser[SwitchTarget] =
         (token(
-          (StringBasic <~ "@").? ~ ((NatBasic) ~ ("." ~> NatBasic).*)
-            .examples(knownVersions*) ~ "!".?
+          (StringBasic <~ "@").? ~ ((NatBasic) ~ ("." ~> NatBasic).*) ~ "!".?
         ) || token(StringBasic))
+          .examples(knownVersions*)
           .map {
             case Left(((vendor, (v1, vs)), bang)) =>
               val force = bang.isDefined
@@ -156,7 +159,10 @@ private[sbt] object CrossJava {
         if (spacePresent) version
         else version & spacedFirst(JavaSwitchCommand)
       val verbose = Parser.opt(token(Space ~> "-v"))
-      val optionalCommand = Parser.opt(token(Space ~> matched(state.combinedParser)))
+      val optionalCommand =
+        Parser.opt(
+          token(Space ~> matched(state.combinedParser).examples(JavaSwitchCommandCompletions*))
+        )
       (spacedVersion ~ verbose ~ optionalCommand).map { case v ~ verbose ~ command =>
         SwitchJavaHome(v, verbose.isDefined, command)
       }

main/src/test/scala/sbt/internal/CrossJavaTest.scala

@@ -242,4 +242,15 @@ class CrossJavaTest extends AnyFunSuite with Diagrams {
       assert(version == "temurin@11.0.15")
     }
   }
+
+  test("java++ tab completion list is bounded (#4310)"):
+    val completions = CrossJava.JavaSwitchCommandCompletions
+    assert(completions.nonEmpty, "completion list must not be empty")
+    assert(
+      completions.size <= 20,
+      "completion list must stay small to avoid JLine 'Display all N possibilities?'"
+    )
+    assert(completions.contains("-v"), "completion list should suggest -v")
+    assert(completions.contains("compile"), "completion list should suggest compile")
+    assert(completions.contains("test"), "completion list should suggest test")
 }