Parse +_ and -_ in types as identifiers under -Xsource:3 to support Scala 3.2 placeholder syntax

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

@@ -1140,7 +1140,13 @@ self =>
               else
                 atPos(start)(makeSafeTupleType(inParens(types())))
             case _      =>
-              if (isWildcardType)
+              if ((in.name == raw.PLUS || in.name == raw.MINUS) && lookingAhead(in.token == USCORE)) {
+                val start = in.offset
+                val identName = in.name.encode.append("_").toTypeName
+                in.nextToken()
+                in.nextToken()
+                atPos(start)(Ident(identName))
+              } else if (isWildcardType)
                 wildcardType(in.skipToken())
               else
                 path(thisOK = false, typeOK = true) match {

test/files/neg/variant-placeholders-future.check

@@ -0,0 +1,7 @@
+variant-placeholders-future.scala:2: error: `=`, `>:`, or `<:` expected
+  type -_ = Int // error -_ not allowed as a type def name without backticks
+        ^
+variant-placeholders-future.scala:3: error: `=`, `>:`, or `<:` expected
+  type +_ = Int // error +_ not allowed as a type def name without backticks
+        ^
+2 errors

test/files/neg/variant-placeholders-future.scala

@@ -0,0 +1,4 @@
+object Test {
+  type -_ = Int // error -_ not allowed as a type def name without backticks
+  type +_ = Int // error +_ not allowed as a type def name without backticks
+}

test/files/pos/variant-placeholders-future.scala

@@ -0,0 +1,33 @@
+object Test {
+  type `-_` = Int
+  type `+_` = Long
+
+  val fnMinusPlus1: -_ => +_ = (_: Int).toLong
+  val fnMinusPlus2: (-_) => +_ = fnMinusPlus1
+  val fnMinusPlus3: -_ => (+_) = fnMinusPlus2
+
+  val fnTupMinusPlus2: (=> -_, -_) => +_ = (a, b) => ((a: Int) + (b: Int)).toLong
+  def defMinusPlus2(byname: => -_, vararg: -_*): +_ = ((vararg.sum: Int) + (byname: -_)).toLong
+  val infixMinusPlus2: -_ Either +_ = Right[-_, +_](1L)
+
+  val optPlus: Option[+_] = Some[ + _ ](1L)  // spaces allowed
+  optPlus match {
+    case opt: Option[ + _ ] =>
+      val opt1: + _ = opt.get
+      val opt2: Long = opt1
+  }
+
+  val optMinus: Option[-_] = Some[ - _ ](1)  // spaces allowed
+  optMinus match {
+    case opt: Option[ - _ ] =>
+      val opt1: `-_` = opt.get
+      val optErr: - _ = opt.get
+      val opt2: Int = opt1
+  }
+
+  locally {
+    type `-_`[A] = A
+    type `+_`[A] = Option[A]
+    val optOpt: Option[ + _ [+_[-_[Int]]]] = Some(Some(Some(1)))
+  }
+}