E212: Only Fully Dependent Applied Constructor Type

This warning is emitted when using an applied constructor type (a type that includes constructor arguments) with a class where not all parameters in the first parameter list are marked as tracked.

Applied constructor types are an experimental feature that allows you to express more precise types by including constructor arguments in the type itself. However, this feature only works correctly when all parameters in the first parameter list are tracked, meaning their values are part of the type.

Example

import scala.language.experimental.modularity

class MyBox(val value: Int)

def example =
  val box: MyBox(42) = MyBox(42)
  box

Error

-- [E212] Type Warning: example.scala:6:11 -------------------------------------
6 |  val box: MyBox(42) = MyBox(42)
  |           ^^^^^^^^^
  |Applied constructor type can only be used with classes where all parameters in the first parameter list are tracked

Solution

Mark the class parameter as tracked to enable applied constructor types:

import scala.language.experimental.modularity

class MyBox(tracked val value: Int)

def example =
  val box: MyBox(42) = MyBox(42)
  box

Alternatively, remove the applied constructor type syntax if you don't need the precise type tracking:

import scala.language.experimental.modularity

class MyBox(val value: Int)

def example =
  val box: MyBox = MyBox(42)
  box