Fix a bug where we don't slide the seed properly for multi-arg forAllF

core/shared/src/main/scala/org/scalacheck/effect/PropF.scala

@@ -66,7 +66,7 @@ sealed trait PropF[F[_]] {
           next.checkOne(Prop.slideSeed(params))
         }
       case PropF.Parameterized(next) =>
-        next(params).checkOne(Prop.slideSeed(params))
+        next(Prop.slideSeed(params)).checkOne(Prop.slideSeed(params))
     }
   }
 

munit/shared/src/test/scala/munit/ScalaCheckEffectSuiteSuite.scala

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2021 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package munit
+
+import cats.effect.IO
+import cats.effect.unsafe.implicits.global
+import org.scalacheck.effect.PropF
+import org.scalacheck.Shrink
+
+// Who tests the tests?
+class ScalaCheckEffectSuiteSuite extends ScalaCheckEffectSuite {
+
+  implicit def noShrink[T]: Shrink[T] = Shrink.shrinkAny[T]
+
+  override def munitValueTransforms: List[ValueTransform] =
+    super.munitValueTransforms ++ List(munitIOTransform)
+
+  // From https://github.com/scalameta/munit/pull/134
+  private val munitIOTransform: ValueTransform =
+    new ValueTransform("IO", { case e: IO[_] => e.unsafeToFuture() })
+
+  test("Correctly slides seed for multi-arg PropF") {
+    var last: Option[Int] = None
+    var duplicates = 0
+
+    PropF.forAllF { (x: Int, y: Int) =>
+      if (Some(x) == last) {
+        duplicates = duplicates + 1
+      }
+      last = Some(y)
+
+      // This is a bit of a heuristic as the generator may genuinely produce identical consecutive values
+      // The bug was that it wasn't sliding so the first generated value was _always_ equal to the second
+      // generated value from the previous iteration
+      IO { assert(clue(duplicates) < 10) }
+    }
+
+  }
+
+}