Deprecate Prop.BooleanOperators in anticipation of Dotty

For a Boolean b and a prop p, the following is ambiguous in dotty:

    import Prop._
    b ==> p

Because Prop contains:

    implicit def BooleanOperators(b: => Boolean) = new ExtendedBoolean(b)
    implicit def propBoolean(b: Boolean): Prop = Prop(b)

And both ExtendedBoolean and Prop provide a `==>` method, the one in
`ExtendedBoolean` just forwards to the one in `Prop`. This is not
ambiguous in scalac because `Boolean` wins against `=> Boolean` but it
is in dotty (and intentionally so).

In general, it seems that all the methods in BooleanOperators are also
available on Prop, so BooleanOperators does not serve any purpose and
can be deprecated. We can then make it non-implicit in a subsequent
release of scalacheck (thus breaking source-compatiblity but not
binary-compatiblity) to finally be able to compile it with Dotty (which
also requires getting typelevel#423 in).

doc/UserGuide.md

@@ -122,7 +122,7 @@ Sometimes, a specification takes the form of an implication. In ScalaCheck,
 you can use the implication operator `==>`:
 
 ```scala
-import org.scalacheck.Prop.{forAll, BooleanOperators}
+import org.scalacheck.Prop.{forAll, propBoolean}
 
 val propMakeList = forAll { n: Int =>
   (n >= 0 && n < 10000) ==> (List.fill(n)("").length == n)
@@ -138,7 +138,7 @@ property holds. In the following trivial example, all cases where `n` is
 non-zero will be thrown away:
 
 ```
-scala> import org.scalacheck.Prop.{forAll, BooleanOperators}
+scala> import org.scalacheck.Prop.{forAll, propBoolean}
 
 scala> val propTrivial = forAll { n: Int =>
      |  (n == 0) ==> (n == 0)
@@ -254,7 +254,7 @@ tell you exactly what part is failing. Look at the following example, where
 the different conditions of the property have been labeled differently:
 
 ```scala
-import org.scalacheck.Prop.{forAll, BooleanOperators}
+import org.scalacheck.Prop.{forAll, propBoolean}
 
 val complexProp = forAll { (m: Int, n: Int) =>
   val res = myMagicFunction(n, m)
@@ -281,7 +281,7 @@ scala> complexProp.check
 It is also possible to write the label before the conditions like this:
 
 ```scala
-import org.scalacheck.Prop.{forAll, BooleanOperators}
+import org.scalacheck.Prop.{forAll, propBoolean}
 
 val complexProp = forAll { (m: Int, n: Int) =>
   val res = myMagicFunction(n, m)
@@ -299,7 +299,7 @@ the value of an intermediate calculation. See the following example, which
 tries to specify multiplication in a somewhat naive way:
 
 ```scala
-import org.scalacheck.Prop.{forAll, BooleanOperators, all}
+import org.scalacheck.Prop.{forAll, propBoolean, all}
 
 val propMul = forAll { (n: Int, m: Int) =>
   val res = n*m

jvm/src/test/scala/org/scalacheck/PropertyFilterSpecification.scala

@@ -9,7 +9,7 @@
 
 package org.scalacheck
 
-import Prop.BooleanOperators
+import Prop.propBoolean
 
 object PropertyFilterSampleSpecification extends Properties("PropertyFilterSample") {
 

jvm/src/test/scala/org/scalacheck/ShrinkSpecificationJVM.scala

@@ -1,6 +1,6 @@
 package org.scalacheck
 
-import Prop.{forAll, BooleanOperators}
+import Prop.{forAll, propBoolean}
 import Shrink.shrink
 import ShrinkSpecification.shrinkClosure
 

jvm/src/test/scala/org/scalacheck/examples/StringUtils.scala

@@ -52,7 +52,7 @@ object StringUtils extends Properties("Examples.StringUtils") {
   }
 
   property("truncate.precond") = Prop.forAll { (s: String, n: Int) =>
-    import Prop.BooleanOperators
+    import Prop.propBoolean
     (n >= 0) ==> {
       val t = StringUtils.truncate(s, n)
       (s.length <= n && t == s) ||

src/main/scala/org/scalacheck/Prop.scala

@@ -353,6 +353,7 @@ object Prop {
   /** Implicit method that makes a number of property operators on boolean
    * values available in the current scope. See [[Prop.ExtendedBoolean]] for
    * documentation on the operators. */
+  @deprecated("Use Prop.propBoolean instead.", since = "ScalaCheck 1.14.1")
   implicit def BooleanOperators(b: => Boolean) = new ExtendedBoolean(b)
 
   /** Implicit conversion of Boolean values to Prop values. */

src/main/scala/org/scalacheck/commands/Commands.scala

@@ -127,7 +127,7 @@ trait Commands {
     /** Wraps the run and postCondition methods in order not to leak the
      *  dependant Result type. */
     private[Commands] def runPC(sut: Sut): (Try[String], State => Prop) = {
-      import Prop.BooleanOperators
+      import Prop.propBoolean
       val r = Try(run(sut))
       (r.map(_.toString), s => preCondition(s) ==> postCondition(s,r))
     }

src/test/scala/org/scalacheck/PropSpecification.scala

@@ -12,7 +12,7 @@ package org.scalacheck
 import Prop.{
   forAll, falsified, undecided, exception, passed, proved, all,
   atLeastOne, sizedProp, someFailing, noneFailing, Undecided, False, True,
-  Exception, Proof, throws, BooleanOperators, secure, delay, lzy
+  Exception, Proof, throws, propBoolean, secure, delay, lzy
 }
 import Gen.{ const, fail, oneOf, listOf, Parameters }
 

src/test/scala/org/scalacheck/ShrinkSpecification.scala

@@ -9,7 +9,7 @@
 
 package org.scalacheck
 
-import Prop.{forAll, forAllNoShrink, BooleanOperators}
+import Prop.{forAll, forAllNoShrink, propBoolean}
 import Shrink.shrink
 
 import scala.concurrent.duration.{Duration, FiniteDuration}

src/test/scala/org/scalacheck/examples/MathSpec.scala

@@ -1,6 +1,6 @@
 package org.scalacheck.examples
 
-import org.scalacheck.Prop.{forAll, BooleanOperators}
+import org.scalacheck.Prop.{forAll, propBoolean}
 
 object MathSpec extends org.scalacheck.Properties("Math") {
   property("sqrt") = forAll { n: Int =>