I got a new exhaustivity warning when switching to 2.10.0-M7 and it seems spurious to me, but it involves existentials so I'm not sure.

To reproduce this warning, we first need to define a sealed abstract class or sealed trait:

scala> sealed abstract class FoundNode[T]
defined class FoundNode

and then tuple it with another parameterized case class, like Some (a fresh case class would also be fine, but Option wouldn't).

scala> object Foo {
     | val v: (Some[_], FoundNode[_]) = (???, ???)
     |   v match {
     |     case (x: Some[t], _) =>
     |   }
     | }
<console>:10: warning: match may not be exhaustive.
It would fail on the following input: (_, _)
         v match {
         ^
defined module Foo

The warning is triggered by matching the first component, of type Some[_], against x: Some[t] (or x: Some[_]). Removing that match removes the warning, as do a lot of other changes. Since the first component has type Some[_], I believe it will always match against Some[_], even if I think of existentials the correct way, so I think the warning is spurious.

Necessary conditions to reproduce this warning seem to be that the scrutinee:

• must be a tuple;
• must have a sealed component (above, FoundNode);
• must have a parametererized case class component (above, Some);
• finally, the matching must include a typed pattern match on the parameterized case class component (above, : Some[_]).

I tried to alter the above example to violate any of those conditions, but that suppresses the warning; I made my experiments available here:
https://github.com/Blaisorblade/scala/blob/583184fa02586765922b6c64846669f345f2c381/test/pending/pos/ExhaustivityWarnBugReportMinimal.scala

Compiler output on the code linked above:

$ ./qbin/scalac test/pending/pos/ExhaustivityWarnBugReportMinimal.scala
test/pending/pos/ExhaustivityWarnBugReportMinimal.scala:19: warning: match may not be exhaustive.
It would fail on the following inputs: (_, FoundFilter()), (_, FoundTypeCase())
  v match {
  ^
test/pending/pos/ExhaustivityWarnBugReportMinimal.scala:22: warning: match may not be exhaustive.
It would fail on the following input: (_, _)
  v match {
  ^
test/pending/pos/ExhaustivityWarnBugReportMinimal.scala:26: warning: match may not be exhaustive.
It would fail on the following input: (_, _)
  v match {
  ^
test/pending/pos/ExhaustivityWarnBugReportMinimal.scala:32: warning: match may not be exhaustive.
It would fail on the following input: (_, _)
  vp match {
  ^
four warnings found