[Misc] Introduce lazyClosure

core/descriptors/src/org/jetbrains/kotlin/types/TypeUtils.kt

@@ -144,6 +144,27 @@ fun <T> Collection<T>.closure(f: (T) -> Collection<T>): Collection<T> {
     return result
 }
 
+fun <T> Collection<T>.lazyClosure(f: (T) -> Collection<T>): Sequence<T> = sequence {
+    if (size == 0) return@sequence
+    var sizeBeforeIteration = 0
+
+    yieldAll(this@lazyClosure)
+    var yieldedCount = size
+    var elementsToCheck = this@lazyClosure
+
+    while (yieldedCount > sizeBeforeIteration) {
+        val toAdd = hashSetOf<T>()
+        elementsToCheck.forEach {
+            val neighbours = f(it)
+            yieldAll(neighbours)
+            yieldedCount += neighbours.size
+            toAdd.addAll(neighbours)
+        }
+        elementsToCheck = toAdd
+        sizeBeforeIteration = yieldedCount
+    }
+}
+
 fun boundClosure(types: Collection<KotlinType>): Collection<KotlinType> =
     types.closure { type -> TypeUtils.getTypeParameterDescriptorOrNull(type)?.upperBounds ?: emptySet() }