Better warning for matching values with flexible types in explicit nulls

[noti0na1]

The code is from discussion in Scala 3.5.0 unreachable case which was working with 3.4.2.

Compiler version

Since 3.5 (after the flexible type was introduced)

Minimized example

-Yexplicit-nulls

def f(s: String) =
  val s2 = s.trim()
  s2 match
    case s3: String => println(1)
    case _ => println(2)

Output Error/Warning message

-- [E030] Match case Unreachable Warning: Stest.scala:1026:7 -------------------
1026 |  case _ => println(2)
     |       ^
     |       Unreachable case
1 warning found

Why this Error/Warning was not helpful

Should produce a better warning similar to without explicit nulls:

-- [E121] Pattern Match Warning: Stest.scala:1026:7 ----------------------------
1026 |  case _ => println(2)
     |       ^
     |Unreachable case except for null (if this is intentional, consider writing case null => instead).
1 warning found

[ajits089]

As per my preliminary investigation what i found is this warning was introduced since 3.0.2. So if we look at the Scala compiler 3.0.0, 3.0.1 and 3.0.2 we receive following response from compiler after defining method f with the given signature and body.
//3.0.2
def f(s: String): Unit -- Warning: 5 | case _ => println(2) | ^ |Unreachable case except for null (if this is intentional, consider writing case null => instead).
// 3.0.1
def f(s: String): Unit 5 | case _ => println(2) | ^ |Unreachable case except for null (if this is intentional, consider writing case null => instead).
//3.0.0
def f(s: String): Unit 5 | case _ => println(2) | ^ | Only null is matched. Consider using case null => instead.

Also, If we consider this case:
def f(s: String | Null) = | val s2 = s.nn.trim() | s2 match | case s3: String => println(1) | case _ => println(2) |
we get warning too where there could be any type involved as the wild card pattern not just null.
1 warning found
def f(s: String | Null): Unit
-- [E121] Pattern Match Warning: -----------------------------------------------
5 | case _ => println(2)
| ^
|Unreachable case except for null (if this is intentional, consider writing case null => instead).