Value + is not a member of Int

[Somainer]

Inspired by Context Functions, I decided to practice my bad taste on simulating the it anonymous function parameter. But it doesn't work.

Minimized code

object Kotlin:
  def it[T](using t: T) = t
  def fun[T, U](fn: T ?=> U)(x: T): U = fn(using x)

import Kotlin.{fun, it}
List(1).map(fun(it + 1))

Output

|List(1).map(fun(it + 1))
  |                ^^^^
  |value + is not a member of Int, but could be made available as an extension method.
  |
  |One of the following imports might fix the problem:
  |
  |  import math.BigDecimal.int2bigDecimal
  |  import math.BigInt.int2bigInt
  |  import math.Numeric.BigDecimalAsIfIntegral.mkNumericOps
  |  import math.Numeric.BigDecimalIsFractional.mkNumericOps
  |  import math.Numeric.BigIntIsIntegral.mkNumericOps
  |  import math.Numeric.IntIsIntegral.mkNumericOps
  |  import Long.long2double
  |  import math.BigDecimal.long2bigDecimal
  |  import Long.long2float
  |  import math.BigInt.long2bigInt
  |

Expectation

This code should compile as it has already been inferred as Int.

Workaround

Providing type arguments to fun works.

List(1).map(fun[Int, Int](it + 1))

[liufengyun]

The following works:

object Kotlin:
  class Ctx[T](val x: T) extends AnyVal

  def fun[T, U](fn: Ctx[T] ?=> U): T => U = (x: T) => fn(using new Ctx(x))
  def it[T](using ctx: Ctx[T]) = ctx.x


import Kotlin._

@main def Test =
  val res = List(1).map(fun(it + 1))
  assert(res == List(2))

[liufengyun]

value + is not a member of Int

The reason is that from the program we only know that T :> Int, which is correct. By wrapping the parameter in a non-variant class Ctx, the compiler may infer T = Int.

[Somainer]

Thank you, it worked.
However, the warning message is very confusing. And I think users like me are not able to find out the reason with the message value + is not a member of Int.

[liufengyun]

We do need to improve the error message. I'll make another PR to fix that.