Fix spacing, i21577.check, and warn comment changes

Author: SuperCl4sh

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -969,12 +969,5 @@ object SpaceEngine {
 
   def checkMatch(m: Match)(using Context): Unit =
     if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
-
-    if reachabilityCheckable(m.selector) then
-      // With explicit nulls, even if the selector type is non-nullable,
-      // we still need to consider the possibility of null value,
-      // so we use the after-erasure nullability for space operations
-      // to achieve consistent runtime behavior.
-      // For example, `val x: String = ???; x match { case null => }` should not be unreachable.
-      withoutMode(Mode.SafeNulls)(checkReachability(m))
+    if reachabilityCheckable(m.selector) then checkReachability(m)
 }

tests/explicit-nulls/neg/flow-match.scala

@@ -3,13 +3,11 @@
 object MatchTest {
   def f6(s: String | Null): String = s match {
     case s2 => s2 // error
-    case null => "other"
-    case s3 => s3
+    case s3 => s3 // OK since not null
   }
 
   def f7(s: String | Null): String = s match {
     case null => "other"
-    case null => "other"
-    case s3 => s3
+    case s3 => s3 // OK since not null
   }
-}
+}

tests/explicit-nulls/neg/pattern-matching.scala

@@ -51,4 +51,4 @@ class Foo:
     case Cat => 200
     case null => 300 // ok
 
-// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)
+// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/explicit-nulls/run/pattern-matching.scala

@@ -34,4 +34,4 @@ object Test:
     val r6 = s2 match
       case null => 200
       case s2: String => 100
-    assert(r6 == 200)
+    assert(r6 == 200)

tests/explicit-nulls/warn/flow-match.check

@@ -0,0 +1,16 @@
+-- [E030] Match case Unreachable Warning: tests/explicit-nulls/warn/flow-match.scala:6:9 -------------------------------
+6 |    case null => "other" // warn
+  |         ^^^^
+  |         Unreachable case
+-- [E030] Match case Unreachable Warning: tests/explicit-nulls/warn/flow-match.scala:7:9 -------------------------------
+7 |    case s3 => s3 // warn
+  |         ^^
+  |         Unreachable case
+-- [E030] Match case Unreachable Warning: tests/explicit-nulls/warn/flow-match.scala:12:9 ------------------------------
+12 |    case null => "other" // warn
+   |         ^^^^
+   |         Unreachable case
+-- [E030] Match case Unreachable Warning: tests/explicit-nulls/warn/flow-match.scala:14:9 ------------------------------
+14 |    case s4 => s4.nn // warn
+   |         ^^
+   |         Unreachable case

tests/explicit-nulls/warn/flow-match.scala

@@ -0,0 +1,16 @@
+// Test unreachable matches in presence of nulls
+
+object MatchTest2 {
+  def f6(s: String | Null): String = s match {
+    case s2 => s2.nn
+    case null => "other" // warn
+    case s3 => s3 // warn
+  }
+
+  def f7(s: String | Null): String = s match {
+    case null => "other"
+    case null => "other" // warn
+    case s3: String => s3
+    case s4 => s4.nn // warn
+  }
+}

tests/explicit-nulls/warn/i21577.check

@@ -1,32 +1,32 @@
--- [E121] Pattern Match Warning: tests\explicit-nulls\warn\i21577.scala:5:9 --------------------------------------------
-5 |    case _ => // warn
-  |         ^
-  |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
--- [E121] Pattern Match Warning: tests\explicit-nulls\warn\i21577.scala:12:9 -------------------------------------------
-12 |    case _ => // warn
-   |         ^
-   |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
--- [E121] Pattern Match Warning: tests\explicit-nulls\warn\i21577.scala:16:7 -------------------------------------------
-16 |  case _ => // warn
-   |       ^
-   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
--- [E121] Pattern Match Warning: tests\explicit-nulls\warn\i21577.scala:20:7 -------------------------------------------
-20 |  case _ => // warn
-   |       ^
-   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
--- [E029] Pattern Match Exhaustivity Warning: tests\explicit-nulls\warn\i21577.scala:29:27 -----------------------------
-29 |def f7(s: String | Null) = s match // warn
-   |                           ^
-   |                           match may not be exhaustive.
-   |
-   |                           It would fail on pattern case: _: Null
-   |
-   | longer explanation available when compiling with `-explain`
--- [E029] Pattern Match Exhaustivity Warning: tests\explicit-nulls\warn\i21577.scala:36:33 -----------------------------
-36 |def f9(s: String | Int | Null) = s match // warn
-   |                                 ^
-   |                                 match may not be exhaustive.
-   |
-   |                                 It would fail on pattern case: _: Int
-   |
-   | longer explanation available when compiling with `-explain`
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:5:9 --------------------------------------------
+5 |    case _ => // warn: null only
+  |         ^
+  |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:12:9 -------------------------------------------
+12 |    case _ => // warn: null only
+   |         ^
+   |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:16:7 -------------------------------------------
+16 |  case _ => // warn: null only
+   |       ^
+   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E030] Match case Unreachable Warning: tests/explicit-nulls/warn/i21577.scala:20:7 ----------------------------------
+20 |  case _ => // warn: unreachable
+   |       ^
+   |       Unreachable case
+-- [E029] Pattern Match Exhaustivity Warning: tests/explicit-nulls/warn/i21577.scala:29:27 -----------------------------
+29 |def f7(s: String | Null) = s match // warn: not exhaustive
+   |                           ^
+   |                           match may not be exhaustive.
+   |
+   |                           It would fail on pattern case: _: Null
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E029] Pattern Match Exhaustivity Warning: tests/explicit-nulls/warn/i21577.scala:36:33 -----------------------------
+36 |def f9(s: String | Int | Null) = s match // warn: not exhaustive
+   |                                 ^
+   |                                 match may not be exhaustive.
+   |
+   |                                 It would fail on pattern case: _: Int
+   |
+   | longer explanation available when compiling with `-explain`

tests/explicit-nulls/warn/i21577.scala

@@ -2,22 +2,22 @@ def f(s: String) =
   val s2 = s.trim()
   s2 match
     case s3: String =>
-    case _ => // warn
+    case _ => // warn: null only
 
 
 def f2(s: String | Null) =
   val s2 = s.nn.trim()
   s2 match
     case s3: String =>
-    case _ => // warn
+    case _ => // warn: null only
 
 def f3(s: String | Null) = s match
   case s2: String =>
-  case _ => // warn
+  case _ => // warn: null only
 
 def f5(s: String) = s match
   case _: String =>
-  case _ => // warn
+  case _ => // warn: unreachable
 
 def f6(s: String) = s.trim() match
   case _: String =>
@@ -26,13 +26,13 @@ def f6(s: String) = s.trim() match
 def f61(s: String) = s.trim() match
   case _: String =>
 
-def f7(s: String | Null) = s match // warn
+def f7(s: String | Null) = s match // warn: not exhaustive
   case _: String =>
 
 def f8(s: String | Null) = s match
   case _: String =>
   case null =>
 
-def f9(s: String | Int | Null) = s match // warn
+def f9(s: String | Int | Null) = s match // warn: not exhaustive
   case _: String =>
-  case null =>
+  case null =>