(implicit conversions + method values) cause strange problem

1.scala

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scala> import scala.collection.immutable.StringLike
import scala.collection.immutable.StringLike

scala> "a".format(_)
res0: Any* => String = <function1>

scala> "a".format(_: String)
res1: String => String = <function1>

scala> "a".format(_: String, _: String)
res2: (String, String) => String = <function2>

scala> "a".format(_: String, _: String, _: Int)
res3: (String, String, Int) => String = <function3>

scala> "a".format _
<console>:9: error: missing arguments for method format in trait StringLike;
follow this method with `_' if you want to treat it as a partially applied function
              "a".format _
                  ^

scala> ("a":StringLike[String]).format _
res5: Any* => String = <function1>

scala> class A
defined class A

scala> class B(a: A) { def f(i: Int) = i }
defined class B

scala> implicit def a(a: A): B = new B(a)
a: (a: A)B

scala> val a = new A
a: A = A@768d26dd

scala> a.f(1)
res11: Int = 1

scala> a.f(_)
res12: Int => Int = <function1>

scala> a.f _
<console>:17: error: missing arguments for method f in class B;
follow this method with `_' if you want to treat it as a partially applied function
              a.f _

scala> (a: B).f _
res15: Int => Int = <function1>

implicit conversion が関係あるみたいですねえ