Deprecate empty-paren (nilary) prefix unary operator

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

@@ -2815,6 +2815,18 @@ self =>
             }
             expr()
           }
+        if (nme.isEncodedUnary(name) && vparamss.nonEmpty) {
+          def instead = DefDef(newmods, name.toTermName.decodedName, tparams, vparamss.drop(1), restype, rhs)
+          def unaryMsg(what: String) = s"unary prefix operator definition with empty parameter list is $what: instead, remove () to declare as `$instead`"
+          def warnNilary(): Unit =
+            if (currentRun.isScala3) syntaxError(nameOffset, unaryMsg("unsupported"))
+            else deprecationWarning(nameOffset, unaryMsg("deprecated"), "2.13.4")
+          vparamss match {
+            case List(List())                               => warnNilary()
+            case List(List(), x :: xs) if x.mods.isImplicit => warnNilary()
+            case _ => // ok
+          }
+        }
         DefDef(newmods, name.toTermName, tparams, vparamss, restype, rhs)
       }
       signalParseProgress(result.pos)

src/reflect/scala/reflect/internal/Printers.scala

@@ -81,6 +81,10 @@ trait Printers extends api.Printers { self: SymbolTable =>
     def indent() = indentMargin += indentStep
     def undent() = indentMargin -= indentStep
 
+    protected def checkForBlank(cond: Boolean) = if (cond) " " else ""
+    protected def blankForOperatorName(name: Name) = checkForBlank(name.isOperatorName)
+    protected def blankForName(name: Name) = checkForBlank(name.isOperatorName || name.endsWith("_"))
+
     def printPosition(tree: Tree) =
       if (printPositions) comment(print(tree.pos.show))
 
@@ -356,7 +360,11 @@ trait Printers extends api.Printers { self: SymbolTable =>
           }
 
         case dd @ DefDef(mods, name, tparams, vparamss, tp, rhs) =>
-          printDefDef(dd, symName(tree, name))(printOpt(": ", tp))(printOpt(" = ", rhs))
+          printDefDef(dd, symName(tree, name)) {
+            // place space after symbolic def name (def !: Unit does not compile)
+            if (tparams.isEmpty && vparamss.isEmpty) printOpt(blankForName(name.encodedName) + ": ", tp)
+            else printOpt(": ", tp)
+          } (printOpt(" = ", rhs))
 
         case td @ TypeDef(mods, name, tparams, rhs) =>
           printTypeDef(td, symName(tree, name))
@@ -593,10 +601,6 @@ trait Printers extends api.Printers { self: SymbolTable =>
       }
     }
 
-    protected def checkForBlank(cond: Boolean) = if (cond) " " else ""
-    protected def blankForOperatorName(name: Name) = checkForBlank(name.isOperatorName)
-    protected def blankForName(name: Name) = checkForBlank(name.isOperatorName || name.endsWith("_"))
-
     protected def resolveSelect(t: Tree): String = {
       t match {
         // case for: 1) (if (a) b else c).meth1.meth2 or 2) 1 + 5 should be represented as (1).+(5)

src/reflect/scala/reflect/internal/StdNames.scala

@@ -988,6 +988,8 @@ trait StdNames {
     val UNARY_- = encode("unary_-")
     val UNARY_! = encode("unary_!")
 
+    val isEncodedUnary = Set[Name](UNARY_~, UNARY_+, UNARY_-, UNARY_!)
+
     // Grouped here so Cleanup knows what tests to perform.
     val CommonOpNames   = Set[Name](OR, XOR, AND, EQ, NE)
     val BooleanOpNames  = Set[Name](ZOR, ZAND, UNARY_!) ++ CommonOpNames

test/files/neg/prefix-unary-nilary-deprecation.check

@@ -0,0 +1,14 @@
+prefix-unary-nilary-deprecation.scala:4: warning: unary prefix operator definition with empty parameter list is deprecated: instead, remove () to declare as `def unary_~ : Foo = this`
+  def unary_~() : Foo = this
+      ^
+prefix-unary-nilary-deprecation.scala:5: warning: unary prefix operator definition with empty parameter list is deprecated: instead, remove () to declare as `def unary_-(implicit pos: Long) = this`
+  def unary_-()(implicit pos: Long) = this
+      ^
+prefix-unary-nilary-deprecation.scala:12: warning: Auto-application to `()` is deprecated. Supply the empty argument list `()` explicitly to invoke method unary_~,
+or remove the empty argument list from its definition (Java-defined methods are exempt).
+In Scala 3, an unapplied method like this will be eta-expanded into a function.
+  val f2 = ~f
+           ^
+error: No warnings can be incurred under -Werror.
+3 warnings
+1 error

test/files/neg/prefix-unary-nilary-deprecation.scala

@@ -0,0 +1,13 @@
+// scalac: -Werror -Xlint:deprecation
+//
+class Foo {
+  def unary_~() : Foo = this
+  def unary_-()(implicit pos: Long) = this
+
+  def unary_! : Foo = this // ok
+  def unary_+(implicit pos: Long) = this // ok
+}
+object Test {
+  val f = new Foo
+  val f2 = ~f
+}

test/files/neg/prefix-unary-nilary-removal.check

@@ -0,0 +1,7 @@
+prefix-unary-nilary-removal.scala:4: error: unary prefix operator definition with empty parameter list is unsupported: instead, remove () to declare as `def unary_~ : Foo = this`
+  def unary_~() : Foo = this
+      ^
+prefix-unary-nilary-removal.scala:5: error: unary prefix operator definition with empty parameter list is unsupported: instead, remove () to declare as `def unary_-(implicit pos: Long) = this`
+  def unary_-()(implicit pos: Long) = this
+      ^
+2 errors

test/files/neg/prefix-unary-nilary-removal.scala

@@ -0,0 +1,13 @@
+// scalac: -Xsource:3
+//
+class Foo {
+  def unary_~() : Foo = this
+  def unary_-()(implicit pos: Long) = this
+
+  def unary_! : Foo = this // ok
+  def unary_+(implicit pos: Long) = this // ok
+}
+object Test {
+  val f = new Foo
+  val f2 = ~f
+}