Binding name to sequence wildcard induces exhaustivity warning

[som-snytt]

Minimized code

scala> sealed trait T ; case class C(x: Int) extends T
// defined trait T
// defined case class C

scala> (C(42): T) match { case C(42) => }
1 |(C(42): T) match { case C(42) => }
  | ^^^^^^^^
  | match may not be exhaustive.
  |
  | It would fail on pattern case: C(_)

scala> (C(42): T) match { case C(_) => }

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

scala> sealed trait T ; case class C(x: Int*) extends T
// defined trait T
// defined case class C

scala> (C(42): T) match { case C(42) => }
1 |(C(42): T) match { case C(42) => }
  | ^^^^^^^^
  | match may not be exhaustive.
  |
  | It would fail on pattern case: C(_, _: _*), C(_, _, _: _*), C()

scala> (C(42): T) match { case C(xs: _*) => }
1 |(C(42): T) match { case C(xs: _*) => }
  | ^^^^^^^^
  | match may not be exhaustive.
  |
  | It would fail on pattern case: C(_: _*)

scala> (C(42): T) match { case C(_: _*) => }

scala>                                                         

Output

scala> (C(42): T) match { case C(xs: _*) => }
1 |(C(42): T) match { case C(xs: _*) => }
  | ^^^^^^^^
  | match may not be exhaustive.
  |
  | It would fail on pattern case: C(_: _*)

Expectation

Expect case C(xs: _*) is as exhaustive as case C(_: _*).

Observed at #8715

[odersky]

Fixed by #8753

[som-snytt]

I see the same behavior on head. I'll update again if I have more info.

sbt:dotty> dotc sandbox/t8757.scala
[warn] Multiple main classes detected.  Run 'show discoveredMainClasses' to see the list
[info] running (fork) dotty.tools.dotc.Main -classpath /Users/andrew/.coursier/cache/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.2/scala-library-2.13.2.jar:/Users/andrew/workspace/dotty/library/../out/bootstrap/dotty-library-bootstrapped/scala-0.25/dotty-library_0.25-0.25.0-bin-SNAPSHOT.jar sandbox/t8757.scala
-- [E029] Pattern Match Exhaustivity Warning: sandbox/t8757.scala:3:17 ---------
3 |def f(): Unit = (C(42): T) match { case C(xs: _*) => }
  |                 ^^^^^^^^
  |                 match may not be exhaustive.
  |
  |                 It would fail on pattern case: C(_: _*)

longer explanation available when compiling with `-explain`
1 warning found
[success] Total time: 5 s, completed Apr 28, 2020, 12:34:06 AM
sbt:dotty> [info] shutting down sbt server
➜  dotty git:(review/exhausted) ✗ cat sandbox/t8757.scala
sealed trait T ; case class C(xs: Int*) extends T

def f(): Unit = (C(42): T) match { case C(xs: _*) => }
def g(): Unit = (C(42): T) match { case C(_: _*) => }

[liufengyun]

This one is not fixed.