Print warning for ()(implicit ...) prefix unary operators too

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

@@ -2816,12 +2816,14 @@ self =>
             expr()
           }
         if (nme.isEncodedUnary(name) && vparamss.nonEmpty) {
-          val tpeStr = if (restype.isEmpty) "" else s" : $restype"
-          def unaryMsg(what: String) = s"unary prefix operator definition with empty parameter list is $what: instead, remove () to declare as `def ${name.decodedName}$tpeStr`"
+          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()) =>
-              if (currentRun.isScala3) syntaxError(nameOffset, unaryMsg("unsupported"))
-              else deprecationWarning(nameOffset, unaryMsg("deprecated"), "2.13.4")
+            case List(List())                               => warnNilary()
+            case List(List(), x :: xs) if x.mods.isImplicit => warnNilary()
             case _ => // ok
           }
         }

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)

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

@@ -1,11 +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`
+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:10: warning: Auto-application to `()` is deprecated. Supply the empty argument list `()` explicitly to invoke method unary_~,
+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.
-2 warnings
+3 warnings
 1 error

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

@@ -2,8 +2,10 @@
 //
 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

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

@@ -1,4 +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`
+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
       ^
-1 error
+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

@@ -2,8 +2,10 @@
 //
 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