Prevent nesting properties

Prevent nesting properties or otherwise adding properties
during execution.

src/main/scala/org/scalacheck/Properties.scala

@@ -29,6 +29,7 @@ import util.ConsoleReporter
 class Properties(val name: String) {
 
   private val props = new scala.collection.mutable.ListBuffer[(String,Prop)]
+  private var frozen = false
 
   /**
    * Customize the parameters specific to this class.
@@ -46,7 +47,10 @@ class Properties(val name: String) {
 
   /** Returns all properties of this collection in a list of name/property
    *  pairs.  */
-  def properties: collection.Seq[(String,Prop)] = props
+  def properties: collection.Seq[(String,Prop)] = {
+    frozen = true // once the properties have been exposed, they must be frozen
+    props
+  }
 
   /** Convenience method that checks the properties with the given parameters
    *  (or default parameters, if not specified)
@@ -100,6 +104,7 @@ class Properties(val name: String) {
    */
   sealed class PropertySpecifier() {
     def update(propName: String, p: => Prop) = {
+      if (frozen) throw new IllegalStateException("cannot nest properties or create properties during execution")
       val fullName = s"$name.$propName"
       props += ((fullName, Prop.delay(p).viewSeed(fullName)))
     }

src/test/scala/org/scalacheck/NoPropertyNestingSpecification.scala

@@ -0,0 +1,37 @@
+/*-------------------------------------------------------------------------*\
+**  ScalaCheck                                                             **
+**  Copyright (c) 2007-2019 Rickard Nilsson. All rights reserved.          **
+**  http://www.scalacheck.org                                              **
+**                                                                         **
+**  This software is released under the terms of the Revised BSD License.  **
+**  There is NO WARRANTY. See the file LICENSE for the full text.          **
+\*------------------------------------------------------------------------ */
+
+package org.scalacheck
+
+object NoPropertyNestingSpecification extends Properties("Properties.no nesting") {
+  property("no nested properties") = {
+    var thrown = false
+
+    val p = new Properties("P") {
+      property("outer") = {
+        property("inner") = true // not allowed!
+        true
+      }
+    }
+
+    p.freeze() // don't have a good way of testing how sbt does this
+    val results = for ((name, prop) <- p.properties) yield prop(Gen.Parameters.default)
+    results match {
+      case collection.Seq(res) => res.status match {
+        case Prop.Exception(e: IllegalStateException) =>
+          if (e.getMessage contains "nest") thrown = true
+          else throw new Exception("exception message did not reference nesting")
+        case _ => throw new Exception("did not get IllegalStateException")
+      }
+      case _ => throw new Exception("more than one property, somehow")
+    }
+
+    thrown == true
+  }
+}