Quasiquotes use Symbol.exists, which isn't (currently) threadsafe in runtime reflection

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[info] ! liftable.lift tuple: Exception raised on property evaluation.
[info] > Exception: java.lang.IllegalArgumentException: requirement failed: Tuples with 16 arity aren't supported

https://scala-ci.typesafe.com/job/scala-2.12.x-integrate-bootstrap/1040/consoleFull

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This failed in the nightly release build for 2.12.x last night. Right before I was planning to cut 2.12.3. Hmmmm.

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    val MaxTupleArity, MaxProductArity, MaxFunctionArity = 22

    lazy val ProductClass          = new VarArityClass("Product", MaxProductArity, countFrom = 1, init = Some(UnitClass))
    lazy val TupleClass            = new VarArityClass("Tuple", MaxTupleArity, countFrom = 1)

...

    class VarArityClass(name: String, maxArity: Int, countFrom: Int = 0, init: Option[ClassSymbol] = None) extends VarArityClassApi {
      private val offset = countFrom - init.size
      private def isDefinedAt(i: Int) = i < seq.length + offset && i >= offset
      val seq: IndexedSeq[ClassSymbol] = (init ++: countFrom.to(maxArity).map { i => getRequiredClass("scala." + name + i) }).toVector
      def apply(i: Int) = if (isDefinedAt(i)) seq(i - offset) else NoSymbol
   }

... 
    object SyntacticTuple extends SyntacticTupleExtractor {
      def apply(args: List[Tree]): Tree = {
        require(args.isEmpty || TupleClass(args.length).exists, s"Tuples with ${args.length} arity aren't supported")
        gen.mkTuple(args)
      }

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Maybe: a new or latent race condition in runtime reflection?

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No sign of that exception with a arity < 22 on google

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I can repro with 2.12.2.

⚡ scala
Welcome to Scala 2.12.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_112).
Type in expressions for evaluation. Or try :help.

scala> import reflect.runtime.universe._; val threads = collection.mutable.Buffer[Thread](); for (i <- 1 to 22; j <- 1 to 8) { val t = new Thread { override def run(): Unit = internal.reificationSupport.SyntacticTuple.apply(List.fill(i)(EmptyTree)) }; threads += t; t.start(); }; threads.foreach(_.join())
Exception in thread "Thread-6" java.lang.IllegalArgumentException: requirement failed: Tuples with 1 arity aren't supported
	at scala.reflect.internal.ReificationSupport$ReificationSupportImpl$SyntacticTuple$.apply(ReificationSupport.scala:430)
	at scala.reflect.internal.ReificationSupport$ReificationSupportImpl$SyntacticTuple$.apply(ReificationSupport.scala:428)
	at $line3.$read$$iw$$iw$$anon$1.run(<console>:11)
Exception in thread "Thread-11" java.lang.ClassCastException: scala.reflect.internal.Types$NoPrefix$ cannot be cast to scala.reflect.internal.Symbols$Symbol
	at scala.reflect.internal.pickling.UnPickler$Scan.readSymbolRef(UnPickler.scala:634)
	at scala.reflect.

I won't hold up the 2.12.3 release on account of this.

The fix will be one or more of:

• GIL synchronize Symbol#exists in SynchronizedSymbol
• simplify Symbol.exists if !isCompilerUniverse (can runtime reflection symbol loaders ever assign NoType? If not, the check collapses to != NoSymbol)
• modify callers of Symbol#exists in ReificationSupport to use != NoSymbol.

/cc @densh @xeno-by in case they have ever run into this, or something that might have the same root cause.