ADd the "DataClassDefaultValues" rule

src/main/kotlin/com/github/ivy/explicit/IvyExplicitRuleSetProvider.kt

@@ -1,5 +1,6 @@
 package com.github.ivy.explicit
 
+import com.github.ivy.explicit.rule.DataClassDefaultValuesRule
 import com.github.ivy.explicit.rule.DataClassFunctionsRule
 import io.gitlab.arturbosch.detekt.api.Config
 import io.gitlab.arturbosch.detekt.api.RuleSet
@@ -13,6 +14,7 @@ class IvyExplicitRuleSetProvider : RuleSetProvider {
             ruleSetId,
             listOf(
                 DataClassFunctionsRule(config),
+                DataClassDefaultValuesRule(config),
             ),
         )
     }

src/main/kotlin/com/github/ivy/explicit/rule/DataClassDefaultValuesRule.kt

@@ -0,0 +1,40 @@
+package com.github.ivy.explicit.rule
+
+import com.github.ivy.explicit.util.Message
+import io.gitlab.arturbosch.detekt.api.*
+import org.jetbrains.kotlin.psi.KtClass
+import org.jetbrains.kotlin.psi.KtParameter
+
+class DataClassDefaultValuesRule(config: Config) : Rule(config) {
+
+    override val issue = Issue(
+        id = "DataClassDefaultValues",
+        severity = Severity.Maintainability,
+        description = "Data class properties should not have default values. " +
+                "Default values lead to implicit instance constructions and problems.",
+        debt = Debt.TWENTY_MINS,
+    )
+
+    override fun visitClass(klass: KtClass) {
+        super.visitClass(klass)
+        if (klass.isData()) {
+            klass.primaryConstructorParameters.filter {
+                it.hasDefaultValue()
+            }.forEach { parameter ->
+                report(
+                    CodeSmell(
+                        issue = issue,
+                        entity = Entity.from(parameter),
+                        message = failureMessage(klass, parameter)
+                    )
+                )
+            }
+        }
+    }
+
+    private fun failureMessage(klass: KtClass, parameter: KtParameter): String = buildString {
+        append("Data class '${klass.name}' should not have default values for properties. ")
+        append("Found default value for property '${Message.parameter(parameter)}'. ")
+        append("This can lead to implicit instance constructions and problems.")
+    }
+}

src/main/kotlin/com/github/ivy/explicit/rule/DataClassFunctionsRule.kt

@@ -1,13 +1,12 @@
 package com.github.ivy.explicit.rule
 
-import com.github.ivy.explicit.util.FunctionMessage
+import com.github.ivy.explicit.util.Message
 import io.gitlab.arturbosch.detekt.api.*
 import io.gitlab.arturbosch.detekt.rules.isOverride
 import org.jetbrains.kotlin.psi.KtClass
 import org.jetbrains.kotlin.psi.KtNamedFunction
 
 class DataClassFunctionsRule(config: Config) : Rule(config) {
-    private val functionMessage: FunctionMessage by lazy { FunctionMessage() }
 
     override val issue = Issue(
         id = "DataClassFunctions",
@@ -44,6 +43,6 @@ class DataClassFunctionsRule(config: Config) : Rule(config) {
     ): String = buildString {
         append("Data class '${klass.name}' should not contain functions. ")
         append("Data classes should only model data and not define behavior. ")
-        append("Found: function '${functionMessage.signature(function)}'.")
+        append("Found: function '${Message.functionSignature(function)}'.")
     }
 }

src/main/kotlin/com/github/ivy/explicit/util/FunctionMessage.kt → src/main/kotlin/com/github/ivy/explicit/util/Message.kt

@@ -1,9 +1,10 @@
 package com.github.ivy.explicit.util
 
 import org.jetbrains.kotlin.psi.KtNamedFunction
+import org.jetbrains.kotlin.psi.KtParameter
 
-class FunctionMessage {
-    fun signature(function: KtNamedFunction): String {
+object Message {
+    fun functionSignature(function: KtNamedFunction): String {
         // Extract the function name
         val fName = function.name ?: ""
 
@@ -28,4 +29,14 @@ class FunctionMessage {
             ": $returnType"
         } else ""
     }
+
+    fun parameter(param: KtParameter): String = buildString {
+        append(param.name)
+        val type = param.typeReference?.text
+        if (type != null) {
+            append(": ")
+            append(type)
+            param.defaultValue?.text?.let { append(" = $it") }
+        }
+    }
 }

src/main/resources/config/config.yml

@@ -1,3 +1,5 @@
 IvyExplicit:
   DataClassFunctions:
     active: true
+  DataClassDefaultValues:
+    active: true

src/test/kotlin/com/github/ivy/explicit/rule/DataClassDefaultValuesRuleTest.kt

@@ -0,0 +1,64 @@
+package com.github.ivy.explicit.rule
+
+import io.gitlab.arturbosch.detekt.api.Config
+import io.gitlab.arturbosch.detekt.rules.KotlinCoreEnvironmentTest
+import io.gitlab.arturbosch.detekt.test.compileAndLintWithContext
+import io.kotest.matchers.collections.shouldHaveSize
+import io.kotest.matchers.shouldBe
+import org.jetbrains.kotlin.cli.jvm.compiler.KotlinCoreEnvironment
+import org.junit.jupiter.api.Test
+
+@KotlinCoreEnvironmentTest
+internal class DataClassDefaultValuesRuleTest(private val env: KotlinCoreEnvironment) {
+
+    @Test
+    fun `reports data class with a default value`() {
+        val code = """
+        data class A(
+            val x: Int = 42
+        )
+        """
+        val findings = DataClassDefaultValuesRule(Config.empty).compileAndLintWithContext(env, code)
+        findings shouldHaveSize 1
+        val message = findings.first().message
+        message shouldBe """
+            Data class 'A' should not have default values for properties. Found default value for property 'x: Int = 42'. This can lead to implicit instance constructions and problems.
+        """.trimIndent()
+    }
+
+    @Test
+    fun `reports data class with a override default value`() {
+        val code = """
+        data class A(
+            override val x: Int = 0,
+            override val y: Int = 0,
+        ): Point
+        """
+        val findings = DataClassDefaultValuesRule(Config.empty).compileAndLintWithContext(env, code)
+        findings shouldHaveSize 2
+    }
+
+    @Test
+    fun `doesn't report class with a default value`() {
+        val code = """
+        class A(
+            val x: Int = 42
+        )
+        """
+        val findings = DataClassDefaultValuesRule(Config.empty).compileAndLintWithContext(env, code)
+        findings shouldHaveSize 0
+    }
+
+    @Test
+    fun `doesn't report data class without default values`() {
+        val code = """
+        class Point(
+            val x: Double,
+            val y: Double,
+        )
+        """
+        val findings = DataClassDefaultValuesRule(Config.empty).compileAndLintWithContext(env, code)
+        findings shouldHaveSize 0
+    }
+
+}