i16018e.scala

// HKT-nested negative coverage for `TypeComparer.compareCaptured` (#16018).
//
// The flat negative matrix in i16018c.scala pins the basic variance
// directions. These cases pin that the same per-position variance checks keep
// firing when the wildcard is nested under an outer covariant container.

object Test:

  class Co[+T]
  class Inv[T]

  // ---- A. Function1: contravariant slot + upper-bounded wildcard ------
  //
  // Outer `Co` covariant descends to `Function1[? <: M, R] <: Function1[M, R]`.
  // The T1 slot has v < 0, so contravariant + upper must not widen.

  def fn_contra_upper_in_co[M, R](xs: Co[Function1[? <: M, R]])
      : Co[Function1[M, R]] = xs                                     // error

  // ---- B. Function1: covariant slot + lower-bounded wildcard ----------
  //
  // Dual to (A). The R slot has v > 0, so covariant + lower must not widen.

  def fn_co_lower_in_co[M, T1](xs: Co[Function1[T1, ? >: M]])
      : Co[Function1[T1, M]] = xs                                    // error

  // ---- C. Invariant slot nested under covariant container -------------
  //
  // Nesting inside a covariant outer container must not smuggle widening
  // past an invariant inner slot.

  def inv_in_co[M](xs: Co[Inv[? <: M]]): Co[Inv[M]] = xs             // error