Monomorphization can't locate default implementations in generic context

[Dimi Racordon]

This program causes the compiler to crash during monomorphization:

public type A: Deinitializable, Strideable {

  public var n: Int

  public memberwise init

  public typealias Stride = Int

  fun offset(to other: Self) -> Stride {
    other.n - self.n
  }

  fun advance(by offset: Stride) -> Self {
    A(n: n + offset)
  }

}

public fun main() {
  let a1 = A(n: 1)
  let a2 = A(n: 2)
  precondition(a1 == a1)
  precondition(a1 != a2) // <- Cause of the crash
  precondition(a1 < a2)
}

Replacing a1 != a2 by !(a1 == a2) silences the problem.