apple/pkl · error · KotlinCodeGeneratorException

Standard library typealias

Error message

Standard library typealias `${qualifiedName}` is not supported by Kotlin code generator. If you think this is an omission, please let us know.

What it means

toKotlinPoetName cannot emit a type reference for a standard library typealias whose aliased type is enum-representable, because stdlib members are not code-generated into the user's output. It throws KotlinCodeGeneratorException asking users to report the omission.

Solutions

1. Replace the stdlib typealias usage in your module with your own string-literal typealias or explicit type.
2. Type the property as String if any of the alias values are acceptable.
3. File an issue with the pkl project asking for support of that specific stdlib typealias.

Example fix

// before (Pkl)
prop: pkl.base.SomeStdlibAlias
// after
typealias MyAlias = "a" | "b"
prop: MyAlias

if (typeAlias.isStandardLibraryMember && CodeGeneratorUtils.isRepresentableAsEnum(typeAlias.aliasedType, null)) {
  throw IllegalArgumentException("Stdlib typealias ${typeAlias.qualifiedName} not supported by Kotlin codegen")
}

fun isStdlibEnumAlias(t: PType.Alias): Boolean =
  t.isStandardLibraryMember && CodeGeneratorUtils.isRepresentableAsEnum(t.aliasedType, null)

try {
  generator.generate(module)
} catch (e: KotlinCodeGeneratorException) {
  if (e.message?.contains("Standard library typealias") == true) {
    logger.warn("Re-type property locally: ${e.message}")
  } else throw e
}

When it happens

Trigger: A user module references a stdlib typealias (isStandardLibraryMember == true) that resolves to an enum-representable aliased type while generating Kotlin code.

Common situations: Properties typed with stdlib type aliases (e.g. hash-algorithm-like literal unions in pkl:base) appearing in modules run through `pkl codegen kotlin`.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Cannot generate Kotlin code for a Pkl standard library…
• Standard library class
• Standard library typealias
• Cannot generate Kotlin enum class for Pkl type alias
• Cannot generate Kotlin enum class for Pkl type alias

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/b345c7c3ba0baff3. Report an issue: GitHub.

Appendix: source

Thrown at pkl-codegen-kotlin/src/main/kotlin/org/pkl/codegen/kotlin/KotlinCodeGenerator.kt:633

    val (packageName, moduleTypeName) = nameMapper.map(moduleName)
    return if (isModuleClass) {
      ClassName(packageName, moduleTypeName)
    } else {
      ClassName(packageName, moduleTypeName, simpleName)
    }
  }

  private fun TypeAlias.toKotlinPoetName(): ClassName {
    val (packageName, moduleTypeName) = nameMapper.map(moduleName)

    return when {
      aliasedType is PType.Alias -> {
        // Kotlin type generated for [this] is a top-level type alias
        ClassName(packageName, simpleName)
      }
      CodeGeneratorUtils.isRepresentableAsEnum(aliasedType, null) -> {
        if (isStandardLibraryMember) {
          throw KotlinCodeGeneratorException(
            "Standard library typealias `${qualifiedName}` is not supported by Kotlin code generator." +
              " If you think this is an omission, please let us know."
          )
        }
        // Kotlin type generated for [this] is a nested enum class
        ClassName(packageName, moduleTypeName, simpleName)
      }
      else -> {
        // Kotlin type generated for [this] is a top-level type alias
        ClassName(packageName, simpleName)
      }
    }
  }

  private fun PType.toKotlinPoetName(): TypeName =
    when (this) {
      PType.UNKNOWN -> ANY_NULL
      PType.NOTHING -> NOTHING

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