Make uninitialized var syntax a migration warning

* In `future-migration` we emit the deprecation warning
* In `future` we emit we make this syntax an error

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

@@ -3654,11 +3654,11 @@ object Parsers {
           subExpr() match
             case rhs0 @ Ident(name) if placeholderParams.nonEmpty && name == placeholderParams.head.name
                 && !tpt.isEmpty && mods.is(Mutable) && lhs.forall(_.isInstanceOf[Ident]) =>
-              if sourceVersion.isAtLeast(future) then
-                deprecationWarning(
-                  em"""`= _` has been deprecated; use `= uninitialized` instead.
-                      |`uninitialized` can be imported with `scala.compiletime.uninitialized`.""",
-                  rhsOffset)
+              report.errorOrMigrationWarning(
+                em"""`= _` has been deprecated; use `= uninitialized` instead.
+                        |`uninitialized` can be imported with `scala.compiletime.uninitialized`.""",
+                in.sourcePos(rhsOffset),
+                from = future)
               placeholderParams = placeholderParams.tail
               atSpan(rhs0.span) { Ident(nme.WILDCARD) }
             case rhs0 => rhs0

tests/neg/uninitialized-future-migration.check

@@ -0,0 +1,5 @@
+-- Error: tests/neg/uninitialized-future-migration.scala:7:15 ----------------------------------------------------------
+7 |  var a: Int = _ // error
+  |               ^
+  |               `= _` has been deprecated; use `= uninitialized` instead.
+  |               `uninitialized` can be imported with `scala.compiletime.uninitialized`.

tests/neg/uninitialized-future-migration.scala

@@ -0,0 +1,8 @@
+//> using options -Werror
+
+import scala.language.`future-migration`
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // error
+  var b: Int = uninitialized

tests/neg/uninitialized-future.scala

@@ -0,0 +1,6 @@
+import scala.language.future
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // error
+  var b: Int = uninitialized

tests/pos/uninitialized-future-migration.scala

@@ -0,0 +1,6 @@
+import scala.language.`future-migration`
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // warn
+  var b: Int = uninitialized