Inconsistent behaviour of reporting illegal cases at the definition site of match types

[noti0na1]

Compiler version

3.4.2-RC1-bin-SNAPSHOT-nonbootstrapped-git-7f410aa

Minimized code

type IllegalMatch1[T] = T match {
  case Int => java.lang.Integer
  case a | Null => a
  case _ => T
}

type IllegalMatch2[T] = T match {
  case a | Null => a
  case _ => T
}

Output

-- [E191] Type Error: Stest.scala:84:26 ----------------------------------------
84 |type IllegalMatch2[T] = T match {
   |                        ^
   |                  The match type contains an illegal case:
   |                      case a | Null => a
   |                  (this error can be ignored for now with `-source:3.3`)
85 |  case a | Null => a
86 |  case _ => T
87 |}

Expectation

case a | Null => ??? is an illegal case based on the specification of match types (#18262).
The illegal cases in both IllegalMatch1 and IllegalMatch2 should be reported at the definition site.

Discussed at: https://users.scala-lang.org/t/updated-to-scala-3-4-0-and-now-getting-compiler-crash/9825/10