Pattern Matching inconsistency

[CyrilFMoser]

Compiler version

3.6.1

Minimized code

This does not issue a pattern match exhaustivity warning:

sealed trait T_A[B]
case class CC_C(a: Int, b: T_A[Int]) extends T_A[Char]

val v_a: T_A[Char] = null
val v_b: Int = v_a match{
  case CC_C(0, _) => 0 
}

But this does:

sealed trait T_A
sealed trait T_B
case class CC_A(a: Int, b: T_B) extends T_A

val v_a: T_A = null
val v_b: Int = v_a match{
  case CC_A(0,_) => 0 
}

Expectation

They should behave the same. The issue generally appears when matching against traits that don't have any case classes extending them, given certain generics. (Here T[Int]). But it does not appear when a trait simply has no case classes extending them, regardless of generics (T_B).

My guess is that this is related to the fact that the pattern matcher does not enforce covering nulls explicitly, so if the only way to create a case class is with a null it thinks that this case class could never be constructed (first program) and therefore also does not need to match against it

I believe my previous bug reports had this same issue at the root, and the fix in #21000 (which to my knowledge should be included in 3.6.1?) did not resolve this issue.

[noti0na1]

Could be related to the discussion at #21934