Fix Arb.list failing within edge cases in case of null values (#3982)

This shall fix the issue as described in
#3981.
Instead of just assuming that a null value means "there is no edge
case", this change adds explicitly checking that, effectively fixing the
problem.

kotest-property/src/commonMain/kotlin/io/kotest/property/arbitrary/collections.kt

@@ -96,13 +96,17 @@ fun <A> Arb.Companion.list(gen: Gen<A>, range: IntRange = 0..100): Arb<List<A>>
       edgecaseFn = { rs ->
          val emptyList = emptyList<A>()
          val singleList: List<A>? = when (gen) {
-            is Arb -> (gen.edgecase(rs) ?: gen.next(rs))?.let { listOf(it) }
-            is Exhaustive -> gen.values.firstOrNull()?.let { listOf(it) }
+            is Arb -> listOf(if (gen.edgecases().isNotEmpty()) gen.edgecases().random(rs.random) else gen.next(rs))
+            is Exhaustive -> gen.values.let { if (it.isEmpty()) null else listOf(it.first()) }
          }
          val repeatedList: List<A>? = when {
             range.last < 2 -> null // too small for repeats
-            gen is Arb -> (gen.edgecase(rs) ?: gen.next(rs))?.let { a -> List(max(2, range.first)) { a } }
-            gen is Exhaustive -> gen.values.firstOrNull()?.let { a -> List(max(2, range.first)) { a } }
+            gen is Arb -> (if (gen.edgecases().isNotEmpty()) gen.edgecases().random(rs.random) else gen.next(rs))
+               .let { a -> List(max(2, range.first)) { a } }
+
+            gen is Exhaustive -> if (gen.values.isEmpty()) null else gen.values.first()
+               .let { a -> List(max(2, range.first)) { a } }
+
             else -> null
          }
          listOfNotNull(emptyList, singleList, repeatedList).filter { it.size in range }.distinct().random(rs.random)

kotest-property/src/jvmTest/kotlin/com/sksamuel/kotest/property/arbitrary/CollectionsTest.kt

@@ -1,28 +1,36 @@
 package com.sksamuel.kotest.property.arbitrary
 
+import io.kotest.assertions.throwables.shouldNotThrowAny
 import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.assertions.throwables.shouldThrowAny
 import io.kotest.core.spec.style.DescribeSpec
 import io.kotest.inspectors.forAll
+import io.kotest.matchers.collections.exist
 import io.kotest.matchers.collections.shouldContain
 import io.kotest.matchers.collections.shouldHaveAtLeastSize
 import io.kotest.matchers.collections.shouldHaveAtMostSize
 import io.kotest.matchers.collections.shouldNotBeEmpty
 import io.kotest.matchers.shouldBe
+import io.kotest.matchers.shouldNot
 import io.kotest.property.Arb
+import io.kotest.property.EdgeConfig
 import io.kotest.property.Exhaustive
+import io.kotest.property.PropTestConfig
 import io.kotest.property.RandomSource
+import io.kotest.property.arbitrary.constant
 import io.kotest.property.arbitrary.double
 import io.kotest.property.arbitrary.edgecases
 import io.kotest.property.arbitrary.int
 import io.kotest.property.arbitrary.list
 import io.kotest.property.arbitrary.of
+import io.kotest.property.arbitrary.orNull
 import io.kotest.property.arbitrary.positiveInt
 import io.kotest.property.arbitrary.set
 import io.kotest.property.arbitrary.single
 import io.kotest.property.arbitrary.take
 import io.kotest.property.checkAll
 import io.kotest.property.exhaustive.constant
+import io.kotest.property.exhaustive.of
 import io.kotest.property.forAll
 
 class CollectionsTest : DescribeSpec({
@@ -91,6 +99,26 @@ class CollectionsTest : DescribeSpec({
             it.shouldHaveAtMostSize(500)
          }
       }
+
+      it("support underlying generators returning null") {
+         checkAll(
+            Exhaustive.of(
+               Exhaustive.constant(null),
+               Arb.constant(null),
+               Arb.constant("").orNull(1.0),
+               Arb.constant(0).orNull(1.0)
+            ),
+            Exhaustive.of(0..100, 1..10, 2..50),
+         ) { gen, range ->
+            shouldNotThrowAny {
+               Arb.list(gen, range).checkAll(PropTestConfig(edgeConfig = EdgeConfig(0.5))) {
+                  it.shouldHaveAtLeastSize(range.first)
+                  it.shouldHaveAtMostSize(range.last)
+                  it shouldNot exist { value -> value != null }
+               }
+            }
+         }
+      }
    }
 
    describe("Arb.set should") {