Refactoring empty type stream

Author: Damtev

usvm-core/src/main/kotlin/org/usvm/types/SupportTypeStream.kt

@@ -43,7 +43,7 @@ class USupportTypeStream<Type> private constructor(
                 if (type == supportType && filtering(type) && typeSystem.isInstantiable(type)) { // exact type
                     USingleTypeStream(typeSystem, type)
                 } else {
-                    UEmptyTypeStream()
+                    emptyTypeStream()
                 }
             }
 
@@ -61,7 +61,7 @@ class USupportTypeStream<Type> private constructor(
     override fun filterByNotSupertype(type: Type): UTypeStream<Type> {
         return when {
             typeSystem.isSupertype(type, supportType) -> {
-                UEmptyTypeStream()
+                emptyTypeStream()
             }
 
             else -> {

usvm-core/src/main/kotlin/org/usvm/types/TypeRegion.kt

@@ -22,7 +22,7 @@ open class UTypeRegion<Type>(
      * Returns region that represents empty set of types. Called when type
      * constraints contradict, for example if X <: Y and X </: Y.
      */
-    protected fun contradiction() = UTypeRegion(typeSystem, UEmptyTypeStream())
+    protected fun contradiction() = UTypeRegion(typeSystem, emptyTypeStream())
     // TODO: generate unsat core for DPLL(T)
 
     /**

usvm-core/src/main/kotlin/org/usvm/types/TypeStream.kt

@@ -53,28 +53,24 @@ interface UTypeStream<Type> {
 /**
  * An empty type stream.
  */
-class UEmptyTypeStream<Type> private constructor() : UTypeStream<Type> {
-    override fun filterBySupertype(type: Type): UTypeStream<Type> = this
+object UEmptyTypeStream : UTypeStream<Nothing> {
+    override fun filterBySupertype(type: Nothing): UTypeStream<Nothing> = this
 
-    override fun filterBySubtype(type: Type): UTypeStream<Type> = this
+    override fun filterBySubtype(type: Nothing): UTypeStream<Nothing> = this
 
-    override fun filterByNotSupertype(type: Type): UTypeStream<Type> = this
+    override fun filterByNotSupertype(type: Nothing): UTypeStream<Nothing> = this
 
-    override fun filterByNotSubtype(type: Type): UTypeStream<Type> = this
+    override fun filterByNotSubtype(type: Nothing): UTypeStream<Nothing> = this
 
-    override fun take(n: Int): Collection<Type> = emptyList()
+    override fun take(n: Int): Collection<Nothing> = emptyList()
 
     override val isEmpty: Boolean
         get() = true
-
-    companion object {
-        @Suppress("UNCHECKED_CAST")
-        operator fun <Type> invoke() = instance as UEmptyTypeStream<Type>
-
-        val instance = UEmptyTypeStream<Nothing>()
-    }
 }
 
+@Suppress("UNCHECKED_CAST")
+fun <Type> emptyTypeStream(): UTypeStream<Type> = UEmptyTypeStream as UTypeStream<Type>
+
 fun <Type> UTypeStream<Type>.takeFirst(): Type = take(1).single()
 
 /**
@@ -86,28 +82,28 @@ class USingleTypeStream<Type>(
 ) : UTypeStream<Type> {
     override fun filterBySupertype(type: Type): UTypeStream<Type> =
         if (!typeSystem.isSupertype(type, singleType)) {
-            UEmptyTypeStream()
+            emptyTypeStream()
         } else {
             this
         }
 
     override fun filterBySubtype(type: Type): UTypeStream<Type> =
         if (!typeSystem.isSupertype(singleType, type)) {
-            UEmptyTypeStream()
+            emptyTypeStream()
         } else {
             this
         }
 
     override fun filterByNotSupertype(type: Type): UTypeStream<Type> =
         if (typeSystem.isSupertype(type, singleType)) {
-            UEmptyTypeStream()
+            emptyTypeStream()
         } else {
             this
         }
 
     override fun filterByNotSubtype(type: Type): UTypeStream<Type> =
         if (typeSystem.isSupertype(singleType, type)) {
-            UEmptyTypeStream()
+            emptyTypeStream()
         } else {
             this
         }