Add config for variable expression in when

detekt-core/src/main/resources/default-detekt-config.yml

@@ -718,6 +718,7 @@ style:
     active: false
   UseIfInsteadOfWhen:
     active: false
+    ignoreWhenContainingVariableDeclaration: false
   UseIsNullOrEmpty:
     active: true
   UseOrEmpty:

detekt-rules-style/src/main/kotlin/io/gitlab/arturbosch/detekt/rules/style/UseIfInsteadOfWhen.kt

@@ -7,6 +7,9 @@ import io.gitlab.arturbosch.detekt.api.Entity
 import io.gitlab.arturbosch.detekt.api.Issue
 import io.gitlab.arturbosch.detekt.api.Rule
 import io.gitlab.arturbosch.detekt.api.Severity
+import io.gitlab.arturbosch.detekt.api.config
+import io.gitlab.arturbosch.detekt.api.internal.Configuration
+import org.jetbrains.kotlin.psi.KtProperty
 import org.jetbrains.kotlin.psi.KtWhenExpression
 
 /**
@@ -34,9 +37,15 @@ class UseIfInsteadOfWhen(config: Config = Config.empty) : Rule(config) {
         Debt.FIVE_MINS
     )
 
+    @Configuration("ignores when statements with a variable declaration used in the subject")
+    private val ignoreWhenContainingVariableDeclaration: Boolean by config(false)
+
+    @Suppress("ReturnCount")
     override fun visitWhenExpression(expression: KtWhenExpression) {
         super.visitWhenExpression(expression)
 
+        if (ignoreWhenContainingVariableDeclaration && expression.subjectExpression is KtProperty) return
+
         if (expression.entries.size == 2 &&
             expression.elseExpression != null &&
             expression.entries.none { it.conditions.size > 1 }

detekt-rules-style/src/test/kotlin/io/gitlab/arturbosch/detekt/rules/style/UseIfInsteadOfWhenSpec.kt

@@ -1,5 +1,6 @@
 package io.gitlab.arturbosch.detekt.rules.style
 
+import io.gitlab.arturbosch.detekt.test.TestConfig
 import io.gitlab.arturbosch.detekt.test.compileAndLint
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
@@ -64,4 +65,48 @@ class UseIfInsteadOfWhenSpec {
         """.trimIndent()
         assertThat(subject.compileAndLint(code)).isEmpty()
     }
+
+    // TC inspired from https://github.com/JetBrains/kotlin/pull/2921/files
+    @Test
+    fun `does not report 'when' variable declaration is present in subject expression when flag is true`() {
+        val code = """
+            enum class Type {
+                HYDRO,
+                PYRO
+            }
+
+            fun select(t: Type) {
+                when (val i = t.ordinal) {
+                    0 -> 1
+                    else -> 42
+                }
+            }
+        """.trimIndent()
+        val subject = UseIfInsteadOfWhen(TestConfig(IGNORE_WHEN_CONTAINING_VARIABLE_DECLARATION to true))
+        assertThat(subject.compileAndLint(code)).isEmpty()
+    }
+
+    // TC inspired from https://github.com/JetBrains/kotlin/pull/2921/files
+    @Test
+    fun `does report 'when' variable declaration is present in subject expression when flag is false`() {
+        val code = """
+            enum class Type {
+                HYDRO,
+                PYRO
+            }
+
+            fun select(t: Type) {
+                when (val i = t.ordinal) {
+                    0 -> 1
+                    else -> 42
+                }
+            }
+        """.trimIndent()
+        val subject = UseIfInsteadOfWhen(TestConfig(IGNORE_WHEN_CONTAINING_VARIABLE_DECLARATION to false))
+        assertThat(subject.compileAndLint(code)).hasSize(1)
+    }
+
+    companion object {
+        private const val IGNORE_WHEN_CONTAINING_VARIABLE_DECLARATION = "ignoreWhenContainingVariableDeclaration"
+    }
 }