FIR: add forgotten ConeIntegerLiteralIntersector (relates to KT-51357)

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/ConeIntegerLiteralIntersector.kt

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2010-2022 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.types
+
+object ConeIntegerLiteralIntersector {
+    fun findCommonIntersectionType(types: Collection<ConeKotlinType>): ConeKotlinType? {
+        if (types.isEmpty()) return null
+        return types.reduce { left: ConeKotlinType?, right: ConeKotlinType? -> fold(left, right) }
+    }
+
+    private fun fold(left: ConeKotlinType?, right: ConeKotlinType?): ConeKotlinType? {
+        if (left == null || right == null) return null
+        return when {
+            left is ConeIntegerLiteralType && right is ConeIntegerLiteralType ->
+                fold(left, right)
+
+            left is ConeIntegerLiteralType -> fold(left, right)
+            right is ConeIntegerLiteralType -> fold(right, left)
+            else -> null
+        }
+    }
+
+    private fun fold(left: ConeIntegerLiteralType, right: ConeIntegerLiteralType): ConeKotlinType? {
+        return when {
+            left.possibleTypes.containsAll(right.possibleTypes) -> right
+            right.possibleTypes.containsAll(left.possibleTypes) -> left
+            else -> null
+        }
+    }
+
+    private fun fold(left: ConeIntegerLiteralType, right: ConeKotlinType): ConeKotlinType? =
+        if (right in left.possibleTypes) right else null
+}

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/ConeTypeIntersector.kt

@@ -69,8 +69,7 @@ object ConeTypeIntersector {
         }
         assert(filteredEqualTypes.isNotEmpty(), errorMessage)
 
-        // TODO
+        ConeIntegerLiteralIntersector.findCommonIntersectionType(filteredEqualTypes)?.let { return it }
-        // IntegerLiteralTypeConstructor.findIntersectionType(filteredEqualTypes)?.let { return it }
 
         /*
          * For the case like it(ft(String..String?), String?), where ft(String..String?) == String?, we prefer to _keep_ flexible type.