[backport] Underscore changes for -Xsource:3: parse +_/-_ and prefix wildcard variables introduced by ?-syntax with ?

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

@@ -722,9 +722,8 @@ self =>
     def isRawBar   = isRawIdent && in.name == raw.BAR
     def isRawIdent = in.token == IDENTIFIER
 
-    def isWildcardType =
-      in.token == USCORE ||
-      currentRun.isScala3 && isRawIdent && in.name == raw.QMARK
+    def isWildcardType = in.token == USCORE || isScala3WildcardType
+    def isScala3WildcardType = currentRun.isScala3 && isRawIdent && in.name == raw.QMARK
 
     def isIdent = in.token == IDENTIFIER || in.token == BACKQUOTED_IDENT
     def isMacro = in.token == IDENTIFIER && in.name == nme.MACROkw
@@ -1077,9 +1076,16 @@ self =>
         simpleTypeRest(in.token match {
           case LPAREN   => atPos(start)(makeSafeTupleType(inParens(types()), start))
           case _        =>
-            if (isWildcardType)
-              wildcardType(in.skipToken())
-            else
+            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) {
+              val scala3Wildcard = isScala3WildcardType
+              wildcardType(in.skipToken(), scala3Wildcard)
+            } else
               path(thisOK = false, typeOK = true) match {
                 case r @ SingletonTypeTree(_) => r
                 case r => convertToTypeId(r)
@@ -1476,8 +1482,8 @@ self =>
      *  WildcardType ::= `_' TypeBounds
      *  }}}
      */
-    def wildcardType(start: Offset) = {
-      val pname = freshTypeName("_$")
+    def wildcardType(start: Offset, qmark: Boolean) = {
+      val pname = if (qmark) freshTypeName("?$") else freshTypeName("_$")
       val t = atPos(start)(Ident(pname))
       val bounds = typeBounds()
       val param = atPos(t.pos union bounds.pos) { makeSyntheticTypeParam(pname, bounds) }
@@ -1989,8 +1995,9 @@ self =>
       def argType(): Tree = {
         val start = in.offset
         if (isWildcardType) {
+            val scala3Wildcard = isScala3WildcardType
             in.nextToken()
-            if (in.token == SUBTYPE || in.token == SUPERTYPE) wildcardType(start)
+            if (in.token == SUBTYPE || in.token == SUPERTYPE) wildcardType(start, scala3Wildcard)
             else atPos(start) { Bind(tpnme.WILDCARD, EmptyTree) }
         } else
           typ() match {
@@ -2357,7 +2364,7 @@ self =>
       val vds   = new ListBuffer[List[ValDef]]
       val start = in.offset
       def paramClause(): List[ValDef] = if (in.token == RPAREN) Nil else {
-        val implicitmod = 
+        val implicitmod =
           if (in.token == IMPLICIT) {
             if (implicitOffset == -1) { implicitOffset = in.offset ; implicitSection = vds.length }
             else if (warnAt == -1) warnAt = in.offset

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/neg/wildcards-future.check

@@ -0,0 +1,11 @@
+wildcards-future.scala:7: error: type mismatch;
+ found   : Map[_$1,_$2] where type _$2 >: Null, type _$1 <: AnyRef
+ required: Map[String,String]
+  underscores : Map[String, String] // error wildcard variables starting with `_`
+  ^
+wildcards-future.scala:9: error: type mismatch;
+ found   : Map[?$1,?$2] where type ?$2 >: Null, type ?$1 <: AnyRef
+ required: Map[String,String]
+  qmarks : Map[String, String] // error – wildcard variables should start with `?` to differentiate from the old syntax
+  ^
+two errors found

test/files/neg/wildcards-future.scala

@@ -0,0 +1,11 @@
+// scalac: -Xsource:3
+//
+object Test {
+  val underscores: Map[_ <: AnyRef, _ >: Null] = Map()
+  val qmarks: Map[? <: AnyRef, ? >: Null] = Map()
+
+  underscores : Map[String, String] // error wildcard variables starting with `_`
+
+  qmarks : Map[String, String] // error – wildcard variables should start with `?` to differentiate from the old syntax
+                               // (and have a mildly more readable error...)
+}

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)))
+  }
+}