How implicit class resolution works ?

ImplicitClassResolutionFail.scala

import org.scalatest.{FunSpec, Matchers}

object Implicits {

  implicit class SeqEitherExtension[E, A](val elt: Seq[Either[E, A]]) extends AnyVal {
    def partition: (Seq[E], Seq[A]) = {
      val (left, right) = elt.partition(_.isLeft)
      (left.collect { case Left(e) => e }, right.collect { case Right(a) => a })
    }
  }

}

class ImplicitsSpec extends FunSpec with Matchers {
  describe("SeqEitherExtension") {
    import Implicits._
    it("partition") {
      // works
      SeqEitherExtension(Seq(Left("err"), Right(1))).partition shouldBe(Seq("err"), Seq(1))

      // do not work, implicit not found :(
      // Any idea on why implicit class is not found ???
      Seq(Left("err"), Right(1)).partition shouldBe(Seq("err"), Seq(1))
      /**
        * Error:(24, 34) missing argument list for method partition in trait TraversableLike
        * Unapplied methods are only converted to functions when a function type is expected.
        * You can make this conversion explicit by writing `partition _` or `partition(_)` instead of `partition`.
        * Seq(Left("err"), Right(1)).partition shouldBe(Seq("err"), Seq(1))
        */
    }
  }
}

from your comment it sounds like the parameterless case is processed under

2. In a selection e.m with e of type T, if the selector mm does not denote an accessible member of T. In this case, a view v is searched which is applicable to e and whose result contains a member named m. The search proceeds as in the case of implicit parameters, where the implicit scope is the one of TT. If such a view is found, the selection e.m is converted to v(e).m.