Allow ? as a wildcard even without -Xsource:3

Like scala#9721, the goal here is to ease
cross-compilation between Scala 2 and 3 code as we prepare for `_` as wildcard
to be deprecated in Scala 3 (cf scala/scala3#5379).

This is technically a source-breaking change, but using a type named ? without
backticks is deprecated since 2.13.6 (scala#9626) and defining a type named ? without
backticks is disallowed since 2.13.7 (scala#9735). The only known user of ? as a type
name was the kind-projector plugin, but support for this syntax was removed from
the plugin a year ago (typelevel/kind-projector#186),
so I believe this change won't be disruptive.

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -713,13 +713,9 @@ self =>
     def isRawIdent = in.token == IDENTIFIER
 
     def isWildcardType = in.token == USCORE || isScala3WildcardType
-    def isScala3WildcardType = settings.isScala3 && isRawIdent && in.name == raw.QMARK
-    def checkQMarkUsage() =
-      if (!settings.isScala3 && isRawIdent && in.name == raw.QMARK)
-        deprecationWarning(in.offset,
-          "Wrap `?` in backticks to continue to use it as an identifier, or use `-Xsource:3` to use it as a wildcard like in Scala 3.", "2.13.6")
+    def isScala3WildcardType = isRawIdent && in.name == raw.QMARK
     def checkQMarkDefinition() =
-      if (isRawIdent && in.name == raw.QMARK)
+      if (isScala3WildcardType)
         syntaxError(in.offset, "using `?` as a type name requires backticks.")
     def checkKeywordDefinition() =
       if (isRawIdent && scala3Keywords.contains(in.name))
@@ -1149,7 +1145,6 @@ self =>
                 val scala3Wildcard = isScala3WildcardType
                 wildcardType(in.skipToken(), scala3Wildcard)
               } else {
-                checkQMarkUsage()
                 path(thisOK = false, typeOK = true) match {
                   case r @ SingletonTypeTree(_) => r
                   case r => convertToTypeId(r)
@@ -2075,7 +2070,6 @@ self =>
             if (in.token == SUBTYPE || in.token == SUPERTYPE) wildcardType(start, scala3Wildcard)
             else atPos(start) { Bind(tpnme.WILDCARD, EmptyTree) }
         } else {
-          checkQMarkUsage()
           typ() match {
             case Ident(name: TypeName) if nme.isVariableName(name) =>
               atPos(start) { Bind(name, EmptyTree) }

test/files/neg/qmark-deprecated.check

@@ -2,7 +2,7 @@ qmark-deprecated.scala:4: error: using `?` as a type name requires backticks.
 class Foo[?] // error
           ^
 qmark-deprecated.scala:6: error: using `?` as a type name requires backticks.
-class Bar[M[?] <: List[?]] // errors
+class Bar[M[?] <: List[?]] // error on the definition
             ^
 qmark-deprecated.scala:10: error: using `?` as a type name requires backticks.
   class ? { val x = 1 } // error
@@ -22,20 +22,4 @@ qmark-deprecated.scala:35: error: using `?` as a type name requires backticks.
 qmark-deprecated.scala:38: error: using `?` as a type name requires backticks.
   type A[?] = Int // error
          ^
-qmark-deprecated.scala:6: warning: Wrap `?` in backticks to continue to use it as an identifier, or use `-Xsource:3` to use it as a wildcard like in Scala 3.
-class Bar[M[?] <: List[?]] // errors
-                       ^
-qmark-deprecated.scala:27: warning: Wrap `?` in backticks to continue to use it as an identifier, or use `-Xsource:3` to use it as a wildcard like in Scala 3.
-  val x: Array[?] = new Array[?](0) // errors
-               ^
-qmark-deprecated.scala:27: warning: Wrap `?` in backticks to continue to use it as an identifier, or use `-Xsource:3` to use it as a wildcard like in Scala 3.
-  val x: Array[?] = new Array[?](0) // errors
-                              ^
-qmark-deprecated.scala:30: warning: Wrap `?` in backticks to continue to use it as an identifier, or use `-Xsource:3` to use it as a wildcard like in Scala 3.
-  def foo1[T <: Array[?]](x: T): Array[?] = x // errors
-                      ^
-qmark-deprecated.scala:30: warning: Wrap `?` in backticks to continue to use it as an identifier, or use `-Xsource:3` to use it as a wildcard like in Scala 3.
-  def foo1[T <: Array[?]](x: T): Array[?] = x // errors
-                                       ^
-5 warnings
 8 errors

test/files/neg/qmark-deprecated.scala

@@ -3,7 +3,7 @@
 
 class Foo[?] // error
 class Foo2[`?`] // ok
-class Bar[M[?] <: List[?]] // errors
+class Bar[M[?] <: List[?]] // error on the definition
 class Bar2[M[`?`] <: List[`?`]] // ok
 
 object G {
@@ -24,10 +24,10 @@ object I {
 object I2 {
   type `?` = Int // ok
 
-  val x: Array[?] = new Array[?](0) // errors
+  val x: Array[?] = new Array[?](0) // no error reported here because we stop before running typer
   val y: Array[`?`] = new Array[`?`](0) // ok
 
-  def foo1[T <: Array[?]](x: T): Array[?] = x // errors
+  def foo1[T <: Array[?]](x: T): Array[?] = x // ok
   def foo2[T <: Array[`?`]](x: T): Array[`?`] = x // ok
 
   def bar1[?] = {} // error

test/files/pos/wildcards-future.scala → test/files/pos/wildcards-present.scala

@@ -1,5 +1,3 @@
-// scalac: -Xsource:3
-//
 object Test {
   val xs: List[?] = List(1, 2, 3)
   val ys: Map[? <: AnyRef, ? >: Null] = Map()