Report illegal codegen parameter types at declaration

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## Unreleased
 
-* None yet.
+* Report illegal `Unit`, `Nothing`, and `Void` codegen constructor parameters on the parameter.
 
 
 ## [2.0.0-alpha.1] - 2026-01-28

moshi-kotlin-codegen/src/main/java/com/squareup/moshi/kotlin/codegen/ksp/TargetTypes.kt

@@ -38,9 +38,12 @@ import com.google.devtools.ksp.symbol.Origin
 import com.squareup.kotlinpoet.AnnotationSpec
 import com.squareup.kotlinpoet.ClassName
 import com.squareup.kotlinpoet.KModifier
+import com.squareup.kotlinpoet.NOTHING
 import com.squareup.kotlinpoet.ParameterizedTypeName.Companion.parameterizedBy
 import com.squareup.kotlinpoet.PropertySpec
 import com.squareup.kotlinpoet.TypeName
+import com.squareup.kotlinpoet.UNIT
+import com.squareup.kotlinpoet.asTypeName
 import com.squareup.kotlinpoet.ksp.TypeParameterResolver
 import com.squareup.kotlinpoet.ksp.toClassName
 import com.squareup.kotlinpoet.ksp.toKModifier
@@ -97,11 +100,7 @@ internal fun targetType(
   val appliedType = AppliedType(type)
 
   val constructor =
-    primaryConstructor(resolver, type, classTypeParamsResolver, logger)
-      ?: run {
-        logger.error("No primary constructor found on $type", type)
-        return null
-      }
+    primaryConstructor(resolver, type, classTypeParamsResolver, logger) ?: return null
   if (constructor.visibility != KModifier.INTERNAL && constructor.visibility != KModifier.PUBLIC) {
     logger.error(
       "@JsonClass can't be applied to $type: " + "primary constructor is not internal or public",
@@ -184,16 +183,32 @@ internal fun primaryConstructor(
   typeParameterResolver: TypeParameterResolver,
   logger: KSPLogger,
 ): TargetConstructor? {
-  val primaryConstructor = targetType.primaryConstructor ?: return null
+  val primaryConstructor =
+    targetType.primaryConstructor
+      ?: run {
+        logger.error("No primary constructor found on $targetType", targetType)
+        return null
+      }
 
   val parameters = LinkedHashMap<String, TargetParameter>()
   for ((index, parameter) in primaryConstructor.parameters.withIndex()) {
     val name = parameter.name!!.getShortName()
+    val typeName = parameter.type.toTypeName(typeParameterResolver)
+    val normalizedTypeName =
+      typeName.unwrapTypeAlias().copy(nullable = false, annotations = emptyList())
+    if (
+      normalizedTypeName == UNIT ||
+        normalizedTypeName == NOTHING ||
+        normalizedTypeName == Void::class.asTypeName()
+    ) {
+      logger.error("Parameter $name with void, Unit, or Nothing type is illegal", parameter)
+      return null
+    }
     parameters[name] =
       TargetParameter(
         name = name,
         index = index,
-        type = parameter.type.toTypeName(typeParameterResolver),
+        type = typeName,
         hasDefault = parameter.hasDefault,
         qualifiers = parameter.qualifiers(resolver),
         jsonName = parameter.jsonName(),

moshi-kotlin-codegen/src/test/java/com/squareup/moshi/kotlin/codegen/ksp/JsonClassSymbolProcessorTest.kt

@@ -570,6 +570,60 @@ class JsonClassSymbolProcessorTest {
     assertThat(result.messages).contains("Error preparing ElementEnvelope")
   }
 
+  @Test
+  fun nullableUnitConstructorParameter() {
+    assertIllegalConstructorParameter("unit", "kotlin.Unit?")
+  }
+
+  @Test
+  fun nullableNothingConstructorParameter() {
+    assertIllegalConstructorParameter("nothing", "kotlin.Nothing?")
+  }
+
+  @Test
+  fun nullableVoidConstructorParameter() {
+    assertIllegalConstructorParameter("void", "java.lang.Void?")
+  }
+
+  @Test
+  fun nestedUnitConstructorParameter() {
+    val result =
+      compile(
+        kotlin(
+          "source.kt",
+          """
+          package test
+          import com.squareup.moshi.JsonClass
+
+          @JsonClass(generateAdapter = true)
+          data class NestedUnit(val units: List<kotlin.Unit>)
+          """,
+        )
+      )
+    assertThat(result.exitCode).isEqualTo(KotlinCompilation.ExitCode.OK)
+  }
+
+  @Test
+  fun chainedUnitTypeAliasConstructorParameter() {
+    val result =
+      compile(
+        kotlin(
+          "source.kt",
+          """
+          package test
+          import com.squareup.moshi.JsonClass
+
+          typealias UnitAlias = kotlin.Unit?
+          typealias ChainedUnitAlias = UnitAlias
+
+          @JsonClass(generateAdapter = true)
+          data class AliasedParameter(val aliasedUnit: ChainedUnitAlias)
+          """,
+        )
+      )
+    assertIllegalConstructorParameter(result, "aliasedUnit")
+  }
+
   @Test
   fun inlineClassWithMultiplePropertiesFails() {
     val result =
@@ -925,4 +979,33 @@ class JsonClassSymbolProcessorTest {
   private fun compile(vararg sourceFiles: SourceFile): JvmCompilationResult {
     return prepareCompilation(*sourceFiles).compile()
   }
+
+  private fun assertIllegalConstructorParameter(parameterName: String, typeName: String) {
+    val result =
+      compile(
+        kotlin(
+          "source.kt",
+          """
+          package test
+          import com.squareup.moshi.JsonClass
+
+          @JsonClass(generateAdapter = true)
+          data class IllegalParameter(val $parameterName: $typeName)
+          """,
+        )
+      )
+    assertIllegalConstructorParameter(result, parameterName)
+  }
+
+  private fun assertIllegalConstructorParameter(
+    result: JvmCompilationResult,
+    parameterName: String,
+  ) {
+    assertThat(result.exitCode).isEqualTo(KotlinCompilation.ExitCode.COMPILATION_ERROR)
+    assertThat(result.messages)
+      .contains("Parameter $parameterName with void, Unit, or Nothing type is illegal")
+    assertThat(result.messages).doesNotContain("Error preparing")
+    assertThat(result.messages).doesNotContain("IllegalStateException")
+    assertThat(result.messages).doesNotContain("No primary constructor found")
+  }
 }