SI-7556 Fix runtime reflection involving ScalaLongSignature

Scala type information is stored in classfiles in encoded in a String
in the ScalaSignature annotation. When it is too big for a single
String, it is split into an array of Strings in a different annotation,
ScalaLongSignature.

The enclosed test, with a class containing 3000 methods, uses the latter.
It exposes a bug in the way runtime reflection decodes that data.

It must concatentate and *then* decode, rather that the other way around.

Author: retronym

src/reflect/scala/reflect/runtime/JavaMirrors.scala

@@ -567,15 +567,10 @@ private[reflect] trait JavaMirrors extends internal.SymbolTable with api.JavaUni
             loadBytes[Array[String]]("scala.reflect.ScalaLongSignature") match {
               case Some(slsig) =>
                 info(s"unpickling Scala $clazz and $module with long Scala signature")
-                val byteSegments = slsig map (_.getBytes)
-                val lens = byteSegments map ByteCodecs.decode
-                val bytes = Array.ofDim[Byte](lens.sum)
-                var len = 0
-                for ((bs, l) <- byteSegments zip lens) {
-                  bs.copyToArray(bytes, len, l)
-                  len += l
-                }
-                unpickler.unpickle(bytes, 0, clazz, module, jclazz.getName)
+                val encoded = slsig flatMap (_.getBytes)
+                val len = ByteCodecs.decode(encoded)
+                val decoded = encoded.take(len)
+                unpickler.unpickle(decoded, 0, clazz, module, jclazz.getName)
               case None =>
                 // class does not have a Scala signature; it's a Java class
                 info("translating reflection info for Java " + jclazz) //debug

test/files/run/t7556.check

@@ -0,0 +1,2 @@
+class annotations: List(scala.reflect.ScalaLongSignature)
+3001 decls via runtime reflection

test/files/run/t7556/Test_2.scala

@@ -0,0 +1,11 @@
+import scala.reflect.runtime.universe._
+
+object Test {
+  def main(args: Array[String]) {
+    val mc = new MegaClass
+    val anns = mc.getClass.getAnnotations.map(_.annotationType.getName).toList.sorted
+    println(s"class annotations: $anns")
+    val N = typeTag[MegaClass].tpe.declarations.size // was:  error reading Scala signature of MegaClass: 65935
+    println(s"$N decls via runtime reflection")
+  }
+}