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scala> class M[A]
defined class M
scala> implicit def ma0[A](a: A): M[A] = null
warning: there was one feature warning; for details, enable `:setting -feature' or `:replay -feature'
ma0: [A](a: A)M[A]
scala> implicit def ma1[A](a: A): M[A] = null
warning: there was one feature warning; for details, enable `:setting -feature' or `:replay -feature'
ma1: [A](a: A)M[A]
scala> def convert[F[X <: F[X]]](builder: F[_ <: F[_]]) = 0
warning: there was one feature warning; for details, enable `:setting -feature' or `:replay -feature'
convert: [F[X <: F[X]]](builder: F[_ <: F[_]])Int
scala> convert(Some[Int](0))
<console>:15: error: no type parameters for method convert: (builder: F[_ <: F[_]])Int exist so that it can be applied to arguments (Some[Int])
--- because ---
argument expression's type is not compatible with formal parameter type;
found : Some[Int]
required: ?F[_$1] forSome { type _$1 <: ?F[_$2] forSome { type _$2 } }
convert(Some[Int](0))
^
<console>:15: error: type mismatch;
found : Some[Int]
required: F[_ <: F[_]]
convert(Some[Int](0))
^
scala> Range(1,2).toArray: Seq[_]
<console>:14: error: polymorphic expression cannot be instantiated to expected type;
found : [B >: Int]Array[B]
required: Seq[_]
Range(1,2).toArray: Seq[_]
^
scala> 0
res2: Int = 0
scala> :quit
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