Scala: Implicit conversion of a function doesn't work

I have this trait with a single method of type WSResponse => HTTPCallResult[A]

trait Parsable[A] {
  def parse(response: WSResponse): HTTPCallResult[A]
}

I want to define an implicit conversion from a function WSResponse => A

object Parsable {
  implicit class DefaultResponseParser[A](parser: WSResponse => A) extends Parsable[A] {
    override def parse(r: WSResponse): HTTPCallResult[A] = {
     //implementation
    }
  }
}

The desired behaviour is such, that if a function WSResponse => HTTPCallResult[A] is passed the anonymous class of Parsable[A] is automatically created. If a function of any other return type is passed (WSResponse => A) the implicit conversion is triggered.

However, an error occurs when compiling with SBT

[error] /Auth.scala:24:60: type mismatch;
[error]  found   : String
[error]  required: HTTPCall.HTTPCallResult[?]
[error]     (which expands to)  scala.util.Either[String,?]
[error]       .post((r: WSResponse) => (r.json \ "key").as[String])

This function was used in order to test it.

def post[A](parser: Parsable[A])
.post((r: WSResponse) => (r.json \ "key").as[String])

EDIT 1: Even making the implicit class an implicit method and moving it to the very same scope as post didn't help.

EDIT 2: It seems that the problem stems from the use of trait with single undefined method. Therefore when passed a function to the post method, the compiler tries to create an anonymous class of type Parser[A] from that. Due to a type missmatch it throws, without even trying to be looking for any implicits.