Experiment: refine macro in Scala 3

modules/core/shared/src/main/scala-3.0+/eu/timepit/refined/package.scala

@@ -14,6 +14,48 @@ package object refined {
    */
   def refineV[P]: RefinePartiallyApplied[Refined, P] = RefType.refinedRefType.refine[P]
 
+  trait Predicate[T, P] {
+    inline def isValid(inline t: T): Boolean
+  }
+
+  class Positive1
+  object Positive1 {
+    inline given Predicate[Int, Positive1] with
+      inline def isValid(inline t: Int): Boolean = t > 0
+  }
+
+  class Greater1[N]
+  object Greater1 {
+    inline given [N <: Int]: Predicate[Int, Greater1[N]] with
+      inline def isValid(inline t: Int): Boolean = t > scala.compiletime.constValue[N]
+  }
+/*
+  Does not work with 3.0.0:
+  scala> refineMV[Int, Greater1[2]](5)
+1 |refineMV[Int, Greater1[2]](5)
+  |^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |Cannot reduce `inline if` because its condition is not a constant value: {
+  |  val given_Predicate_Int_Greater1_this:
+  |    eu.timepit.refined.Greater1.given_Predicate_Int_Greater1[(2 : Int)]
+  |   = eu.timepit.refined.Greater1.given_Predicate_Int_Greater1[(2 : Int)]
+  |  true:Boolean
+  |}
+  | This location contains code that was inlined from package.scala:42
+ */
+
+
+  class NonEmpty1
+  object NonEmpty1 {
+    inline given Predicate[String, NonEmpty1] with
+      inline def isValid(inline s: String): Boolean = !(s == "")
+    // If we use !s.isEmpty here, we get the following compile error:
+    // |Cannot reduce `inline if` because its condition is not a constant value: "hello".isEmpty().unary_! :Boolean
+  }
+
+  inline def refineMV[T, P](inline t: T)(using inline p: Predicate[T, P]): Refined[T, P] = {
+    inline if (p.isValid(t)) Refined.unsafeApply(t) else scala.compiletime.error("no")
+  }
+
   /**
    * Alias for `[[api.RefType.refine]][P]` with `shapeless.tag.@@` as type
    * parameter for `[[api.RefType]]`.