Fix #13440: Avoid capturing var in message closure

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -247,8 +247,9 @@ object Scanners {
       else keyword
 
     private def treatAsIdent(): Token =
+      val name0 = name  // don't capture the `name` var in the message closure, it may be null later
       report.errorOrMigrationWarning(
-        i"$name is now a keyword, write `$name` instead of $name to keep it as an identifier",
+        i"$name0 is now a keyword, write `$name0` instead of $name0 to keep it as an identifier",
         sourcePos())
       patch(source, Span(offset), "`")
       patch(source, Span(offset + name.length), "`")

tests/neg-custom-args/fatal-warnings/i13440.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg-custom-args/fatal-warnings/i13440.scala:3:13 -------------------------------------------------------
+3 |case class A(enum: List[Int] = Nil) // error
+  |             ^
+  |             enum is now a keyword, write `enum` instead of enum to keep it as an identifier

tests/neg-custom-args/fatal-warnings/i13440.scala

@@ -0,0 +1,3 @@
+import language.`3.0-migration`
+
+case class A(enum: List[Int] = Nil) // error