Parse +_ and -_ in types as identifiers to support Scala 3.2 plac…

…eholder syntax

This change allows `kind-projector` plugin to rewrite `+_` and `-_` tokens to type lambdas,
in line with proposed syntax for Scala 3.2 in http://dotty.epfl.ch/docs/reference/changed-features/wildcards.html

When used in conjunction with `-Xsource:3` this will let the user use `?` for wildcards and `_` for placeholders, letting the user cross-compile the same sources with Scala 3 with `-source:3.2` flag.

This change is not source breaking since currently `+_` and `-_` fail to parse entirely,
this change also does not allow the user to declare types with these names without backticks,
they can only be used as part of a type tree.

Gate `-_`/`+_` parsing behind `-Xsource:3` to guarantee minimal disruption to existing code

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

@@ -1077,7 +1077,14 @@ self =>
         simpleTypeRest(in.token match {
           case LPAREN   => atPos(start)(makeSafeTupleType(inParens(types()), start))
           case _        =>
-            if (isWildcardType)
+            if (currentRun.isScala3 && (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:4: error: `=', `>:', or `<:' expected
+  type -_ = Int // error -_ not allowed as a type def name without backticks
+        ^
+variant-placeholders-future.scala:5: error: `=', `>:', or `<:' expected
+  type +_ = Int // error +_ not allowed as a type def name without backticks
+        ^
+two errors found

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

@@ -0,0 +1,6 @@
+// scalac: -Xsource:3
+//
+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/neg/variant-placeholders-nofuture.check

@@ -0,0 +1,7 @@
+variant-placeholders-nofuture.scala:5: error: ';' expected but '_' found.
+  val fnMinusPlus1: -_ => +_ = (_: Int).toLong // error -_/+_ won't parse without -Xsource:3
+                     ^
+variant-placeholders-nofuture.scala:6: error: ')' expected but '_' found.
+  val fnMinusPlus2: (-_) => +_ = fnMinusPlus1  // error -_/+_ won't parse without -Xsource:3
+                      ^
+two errors found

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

@@ -0,0 +1,8 @@
+object Test {
+  type `-_` = Int
+  type `+_` = Long
+
+  val fnMinusPlus1: -_ => +_ = (_: Int).toLong // error -_/+_ won't parse without -Xsource:3
+  val fnMinusPlus2: (-_) => +_ = fnMinusPlus1  // error -_/+_ won't parse without -Xsource:3
+  val fnMinusPlus3: -_ => (+_) = fnMinusPlus2  // error -_/+_ won't parse without -Xsource:3
+}

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

@@ -0,0 +1,35 @@
+// scalac: -Xsource:3
+//
+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)))
+  }
+}