Using -Xprint:typer to see the result - looks like the typer is the one responsible for filling in implicits

typer.scala

/*
$ echo 'scalacOptions += "-Xprint:typer"' > build.sbt
$ sbt console
*/

// with the generic version

scala> class A { def foo[T](): String = ??? }
scala> implicit class PimpedA { def foo[T](i: String): String = ??? }
scala> val a = new A()
scala> a.foo("aoeu")

private[this] val res0: <error> = /*$line5.$read.$iw.$iw.*/a.foo[Nothing]("aoeu");

// with the non-generic version

scala> class B { def foo(): String = ??? }
scala> implicit class PimpedB { def foo(i: String): String = ??? }
scala> val b = new B()
scala> b.foo("aoeu")

private[this] val res1: String = /*$line8.$read.$iw.$iw.*/PimpedB(/*$line9.$read.$iw.$iw.*/b).foo("aoeu");

/*
It looks like the typer is the part of the compiler responsible for filling
in implicit conversions like this, and it's just not resolving when it's 
the generic version.
*/