case class misbehaves with only implicit parameter list

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Comment stolen from paulp’s explanation:

It should fail the compile with error: case classes without a parameter list are not allowed but the implicit parameter list fools it. Then it proceeds to act in a very confused manner, for instance turning all the explicitly written implicit parameters into case accessors.

scala> case class Bip(implicit x: Int, y: Int)
defined class Bip

scala> Bip()(1, 2).productArity
res4: Int = 2

But naturally it eventually inserts the empty parameter list, as seen above. Whereas:

scala> case class Bip2()(implicit x: Int, y: Int)
defined class Bip2

scala> Bip2()(1, 2).productArity
res5: Int = 0

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Imported From: https://issues.scala-lang.org/browse/SI-10097?orig=1
Reporter: Greg Pfeil (sellout)
Affected Versions: 2.12.0
See #9494
Duplicates #8643

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@som-snytt said:
Inconsistent, yet it's almost a feature, except for pattern matching:

scala> case class C(implicit c: Int)
defined class C

scala> implicit val i: Int = 42
i: Int = 42

scala> C() match { case x @ C() => x }
res0: C = C(42)

scala> C() match { case x @ C(i) => i }
<console>:15: error: wrong number of arguments for pattern C()
       C() match { case x @ C(i) => i }
                             ^

[scabug]

@som-snytt said:
scala/scala#5585