Scala 3/Reference/Error codes/E156: Modifier Not Allowed For Definition

E156: Modifier Not Allowed For Definition

This error occurs when a modifier is used on a definition where it is not applicable. Different kinds of definitions (classes, objects, methods, etc.) support different sets of modifiers.

Common invalid combinations include:

opaque on a method definition (only valid for type aliases)
abstract on an object definition
sealed on an object definition

Example

object Test {
  opaque def example: Int = 42
}

Error

-- [E156] Syntax Error: example.scala:2:13 -------------------------------------
2 |  opaque def example: Int = 42
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |  Modifier opaque is not allowed for this definition

Solution

object Test {
  // opaque is only valid for type aliases
  opaque type PositiveInt = Int

  // Regular method definition
  def example: Int = 42
}

// abstract is not allowed on object - use trait or abstract class instead
abstract class Base {
  def example: Int
}

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