[Gradle] Refactor getVisibleSourceSetsFromAssociateCompilations

To highlight the fact that there is an intersection of
visible source sets of compilations where given source set participates

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/sources/SourceSetsVisibilityInference.kt

@@ -36,9 +36,7 @@ internal fun getVisibleSourceSetsFromAssociateCompilations(
     return when (visibleInCompilations.size) {
         0 -> emptySet()
         1 -> visibleInCompilations.single()
-        else -> visibleInCompilations.first().toMutableSet().apply {
+        else -> visibleInCompilations.reduce { intersection, kotlinSourceSets -> intersection intersect kotlinSourceSets }
-            visibleInCompilations.subList(1, visibleInCompilations.size).forEach { retainAll(it) }
-        }
     }.toList()
 }
 

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/unitTests/sources/SourceSetVisibilityFromAssociatedCompilationsTest.kt

@@ -10,7 +10,6 @@ package org.jetbrains.kotlin.gradle.unitTests.sources
 import org.jetbrains.kotlin.gradle.dsl.multiplatformExtension
 import org.jetbrains.kotlin.gradle.plugin.KotlinSourceSet
 import org.jetbrains.kotlin.gradle.plugin.sources.getVisibleSourceSetsFromAssociateCompilations
-import org.jetbrains.kotlin.gradle.plugin.sources.internal
 import org.jetbrains.kotlin.gradle.util.buildProjectWithMPP
 import org.jetbrains.kotlin.util.capitalizeDecapitalize.decapitalizeAsciiOnly
 import kotlin.test.Test
@@ -154,17 +153,56 @@ class SourceSetVisibilityFromAssociatedCompilationsTest {
         "jvmIntegrationTest".assertVisibleSourceSetsFromAssociatedCompilations("commonMain", "jvmMain", "commonTest", "jvmTest")
     }
 
+    @Test
+    fun testVisibleSourceSetsIntersection() {
+        /* Given Following Project Structure:
+              commonTest
+                  |
+               nativeTest
+                  |
+               linuxTest
+                /       \
+           linuxX64Test  linuxArm64Test -associatedWith-> linuxArm64Main2
+
+           by default each ${x}Test source sees ${x}Main and its transitive dependencies.
+        */
+        kotlin.linuxArm64 {
+            val test = compilations.getByName("test")
+            val main2 = compilations.create("main2")
+            test.associateWith(main2)
+        }
+        kotlin.linuxX64()
+
+        // Assert that linuxTest can see only source sets that
+        // *ALL* its underlying compilations (linuxX64Test, linuxArm64Test)
+        // can see as well (i.e. intersection of their visible source sets)
+        "linuxX64Test".assertVisibleSourceSetsFromAssociatedCompilations(
+            "commonMain",
+            "nativeMain",
+            "linuxMain",
+            "linuxX64Main", // unique for given source set
+        )
+        "linuxArm64Test".assertVisibleSourceSetsFromAssociatedCompilations(
+            "commonMain",
+            "nativeMain",
+            "linuxMain",
+            "linuxArm64Main", // unique for given source set
+            "linuxArm64Main2" // unique for given source set
+        )
+        "linuxTest".assertVisibleSourceSetsFromAssociatedCompilations("commonMain", "nativeMain", "linuxMain")
+    }
+
     private fun String.assertVisibleSourceSetsFromAssociatedCompilations(
         vararg expectedVisibleSourceSets: String
     ) = assertEquals(
         expectedVisibleSourceSets.toSet(),
-        getVisibleSourceSetsFromAssociateCompilations(kotlin.sourceSets.getByName(this).internal.compilations).map { it.name }.toSet()
+        getVisibleSourceSetsFromAssociateCompilations(kotlin.sourceSets.getByName(this)).map { it.name }.toSet()
     )
 
     private fun KotlinSourceSet.assertVisibleSourceSetsFromAssociatedCompilations(
         vararg expectedVisibleSourceSets: KotlinSourceSet
     ) = assertEquals(
         expectedVisibleSourceSets.map { it.name }.toSet(),
-        getVisibleSourceSetsFromAssociateCompilations(this.internal.compilations).map { it.name }.toSet()
+        getVisibleSourceSetsFromAssociateCompilations(this).map { it.name }.toSet()
     )
 }