Bugfix for fix mode of AVOID_NULL_CHECK

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter4/NullChecksRule.kt

@@ -149,15 +149,15 @@ class NullChecksRule(configRules: List<RulesConfig>) : DiktatRule(
 
         val text = "$thenEditedCodeLines $elseEditedCodeLines"
         val tree = KotlinParser().createNode(text)
-        condition.treeParent.treeParent.addChild(tree, condition.treeParent)
-        condition.treeParent.treeParent.removeChild(condition.treeParent)
+        val ifNode = condition.treeParent
+        ifNode.treeParent.replaceChild(ifNode, tree)
     }
 
     private fun getEditedElseCodeLines(elseCodeLines: List<String>?, numberOfStatementsInElseBlock: Int): String = when {
         // else { "null"/empty } -> ""
         elseCodeLines == null || elseCodeLines.singleOrNull() == "null" -> ""
         // else { bar() } -> ?: bar()
-        numberOfStatementsInElseBlock == 1 -> "?: ${elseCodeLines.joinToString(postfix = "\n", separator = "\n")}"
+        numberOfStatementsInElseBlock == 1 && elseCodeLines.singleOrNull()?.hasAssignment() != true -> "?: ${elseCodeLines.joinToString(postfix = "\n", separator = "\n")}"
         // else { ... } -> ?: run { ... }
         else -> getDefaultCaseElseCodeLines(elseCodeLines)
     }

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/utils/StringUtils.kt

@@ -12,6 +12,8 @@ internal const val SPACE = ' '
 
 internal const val TAB = '\t'
 
+private const val REGEX_FOR_ASSIGNMENT_DETECTION = "[^=]+=[^=]+"
+
 @Suppress("VARIABLE_NAME_INCORRECT_FORMAT")
 val JAVA = arrayOf("abstract", "assert", "boolean",
     "break", "byte", "case", "catch", "char", "class", "const",
@@ -37,6 +39,11 @@ val KOTLIN = KtTokens.KEYWORDS
  */
 val loggerPropertyRegex = "(?iu)^log(?:ger)?$".toRegex()
 
+/**
+ * @return true if this String includes assignment operator, false otherwise
+ */
+fun String.hasAssignment(): Boolean = contains(REGEX_FOR_ASSIGNMENT_DETECTION.toRegex())
+
 /**
  * @return whether [this] string represents a Java keyword
  */

diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/smoke/DiktatSmokeTestBase.kt

@@ -311,7 +311,7 @@ abstract class DiktatSmokeTestBase : FixTestBase("test/smoke/src/main/kotlin",
 
     companion object {
         const val DEFAULT_CONFIG_PATH = "../diktat-analysis.yml"
-        private const val TEST_TIMEOUT_SECONDS = 20L
+        private const val TEST_TIMEOUT_SECONDS = 25L
         val unfixedLintErrors: MutableList<LintError> = mutableListOf()
 
         // by default using same yml config as for diktat code style check, but allow to override

diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckExpected.kt

@@ -79,3 +79,25 @@ foo()
 } ?: boo()
 }
 
+fun nullCheckWithAssumption() {
+    val a: Int? = 5
+    a?.let {
+foo()
+} ?: run {
+a = 5
+}
+    a?.let {
+foo()
+} ?: run {
+a = 5
+}
+    a?.let {
+a = 5
+} ?: foo()
+    a?.let {
+a = 5
+} ?: foo()
+    a?.let {
+foo()
+} ?: a == 5
+}

diktat-rules/src/test/resources/test/paragraph4/null_checks/IfConditionNullCheckTest.kt

@@ -111,3 +111,31 @@ fun reversedCheckSmartCases() {
     }
 }
 
+fun nullCheckWithAssumption() {
+    val a: Int? = 5
+    if (a != null) {
+        foo()
+    } else {
+        a = 5
+    }
+    if (a == null) {
+        a = 5
+    } else {
+        foo()
+    }
+    if (a != null) {
+        a = 5
+    } else {
+        foo()
+    }
+    if (a == null) {
+        foo()
+    } else {
+        a = 5
+    }
+    if (a != null) {
+        foo()
+    } else {
+        a == 5
+    }
+}