From 3240841c893131ff40cd1ce09bf3e6097d302d09 Mon Sep 17 00:00:00 2001
From: Eugene Yokota <eed3si9n@gmail.com>
Date: Wed, 24 Jun 2020 19:14:34 -0400
Subject: [PATCH 1/3] Deprecate empty-paren (nilary) prefix unary operator

Ref scala/bug 12055

> Now that auto-application is deprecated, a prefix with a nilary creates a warning that cannot be avoided while maintaining a prefix position (or permanently applying warning suppression).

This adds the following warning

```
deprecation.scala:4: warning: empty-paren (nilary) prefix unary operator is deprecated: instead, remove () to declare as `def unary_~ : Foo`
```

I've opted to use the term "empty-paren" as defined in https://www.artima.com/pins1ed/composition-and-inheritance.html.
---
 src/compiler/scala/tools/nsc/ast/parser/Parsers.scala | 10 ++++++++++
 src/reflect/scala/reflect/internal/StdNames.scala     |  2 ++
 test/files/neg/prefix-unary-nilary-deprecation.check  | 11 +++++++++++
 test/files/neg/prefix-unary-nilary-deprecation.scala  | 11 +++++++++++
 test/files/neg/prefix-unary-nilary-removal.check      |  4 ++++
 test/files/neg/prefix-unary-nilary-removal.scala      | 11 +++++++++++
 6 files changed, 49 insertions(+)
 create mode 100644 test/files/neg/prefix-unary-nilary-deprecation.check
 create mode 100644 test/files/neg/prefix-unary-nilary-deprecation.scala
 create mode 100644 test/files/neg/prefix-unary-nilary-removal.check
 create mode 100644 test/files/neg/prefix-unary-nilary-removal.scala

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index 78bc308079e3..ca796fc46050 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
@@ -2815,6 +2815,16 @@ self =>
             }
             expr()
           }
+        if (nme.isEncodedUnary(name) && vparamss.nonEmpty) {
+          val tpeStr = if (restype.isEmpty) "" else s" : $restype"
+          def unaryMsg(what: String) = s"empty-paren (nilary) prefix unary operator is $what: instead, remove () to declare as `def ${name.decodedName}$tpeStr`"
+          vparamss match {
+            case List(List()) =>
+              if (currentRun.isScala3) syntaxError(nameOffset, unaryMsg("unsupported"))
+              else deprecationWarning(nameOffset, unaryMsg("deprecated"), "2.13.4")
+            case _ => // ok
+          }
+        }
         DefDef(newmods, name.toTermName, tparams, vparamss, restype, rhs)
       }
       signalParseProgress(result.pos)
diff --git a/src/reflect/scala/reflect/internal/StdNames.scala b/src/reflect/scala/reflect/internal/StdNames.scala
index e54fdd290932..15cdeea2a155 100644
--- a/src/reflect/scala/reflect/internal/StdNames.scala
+++ b/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
diff --git a/test/files/neg/prefix-unary-nilary-deprecation.check b/test/files/neg/prefix-unary-nilary-deprecation.check
new file mode 100644
index 000000000000..caf40a59225b
--- /dev/null
+++ b/test/files/neg/prefix-unary-nilary-deprecation.check
@@ -0,0 +1,11 @@
+prefix-unary-nilary-deprecation.scala:4: warning: empty-paren (nilary) prefix unary operator is deprecated: instead, remove () to declare as `def unary_~ : Foo`
+  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_~,
+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
+1 error
diff --git a/test/files/neg/prefix-unary-nilary-deprecation.scala b/test/files/neg/prefix-unary-nilary-deprecation.scala
new file mode 100644
index 000000000000..0dad1f65eb6b
--- /dev/null
+++ b/test/files/neg/prefix-unary-nilary-deprecation.scala
@@ -0,0 +1,11 @@
+// scalac: -Werror -Xlint:deprecation
+//
+class Foo {
+  def unary_~() : Foo = this
+
+  def unary_! : Foo = this // ok
+}
+object Test {
+  val f = new Foo
+  val f2 = ~f
+}
diff --git a/test/files/neg/prefix-unary-nilary-removal.check b/test/files/neg/prefix-unary-nilary-removal.check
new file mode 100644
index 000000000000..1bc3a44c5a23
--- /dev/null
+++ b/test/files/neg/prefix-unary-nilary-removal.check
@@ -0,0 +1,4 @@
+prefix-unary-nilary-removal.scala:4: error: empty-paren (nilary) prefix unary operator is unsupported: instead, remove () to declare as `def unary_~ : Foo`
+  def unary_~() : Foo = this
+      ^
+1 error
diff --git a/test/files/neg/prefix-unary-nilary-removal.scala b/test/files/neg/prefix-unary-nilary-removal.scala
new file mode 100644
index 000000000000..fde5a1043caa
--- /dev/null
+++ b/test/files/neg/prefix-unary-nilary-removal.scala
@@ -0,0 +1,11 @@
+// scalac: -Xsource:3
+//
+class Foo {
+  def unary_~() : Foo = this
+
+  def unary_! : Foo = this // ok
+}
+object Test {
+  val f = new Foo
+  val f2 = ~f
+}

From 12f1c41406a528cb8b98a76f3142a037cf780b9d Mon Sep 17 00:00:00 2001
From: Eugene Yokota <eed3si9n@gmail.com>
Date: Wed, 24 Jun 2020 21:49:48 -0400
Subject: [PATCH 2/3] Update warning message per review

---
 src/compiler/scala/tools/nsc/ast/parser/Parsers.scala | 2 +-
 test/files/neg/prefix-unary-nilary-deprecation.check  | 2 +-
 test/files/neg/prefix-unary-nilary-removal.check      | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index ca796fc46050..1961fceeb230 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
@@ -2817,7 +2817,7 @@ self =>
           }
         if (nme.isEncodedUnary(name) && vparamss.nonEmpty) {
           val tpeStr = if (restype.isEmpty) "" else s" : $restype"
-          def unaryMsg(what: String) = s"empty-paren (nilary) prefix unary operator is $what: instead, remove () to declare as `def ${name.decodedName}$tpeStr`"
+          def unaryMsg(what: String) = s"unary prefix operator definition with empty parameter list is $what: instead, remove () to declare as `def ${name.decodedName}$tpeStr`"
           vparamss match {
             case List(List()) =>
               if (currentRun.isScala3) syntaxError(nameOffset, unaryMsg("unsupported"))
diff --git a/test/files/neg/prefix-unary-nilary-deprecation.check b/test/files/neg/prefix-unary-nilary-deprecation.check
index caf40a59225b..be1eddcbbb15 100644
--- a/test/files/neg/prefix-unary-nilary-deprecation.check
+++ b/test/files/neg/prefix-unary-nilary-deprecation.check
@@ -1,4 +1,4 @@
-prefix-unary-nilary-deprecation.scala:4: warning: empty-paren (nilary) prefix unary operator 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`
   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_~,
diff --git a/test/files/neg/prefix-unary-nilary-removal.check b/test/files/neg/prefix-unary-nilary-removal.check
index 1bc3a44c5a23..ac671f41875c 100644
--- a/test/files/neg/prefix-unary-nilary-removal.check
+++ b/test/files/neg/prefix-unary-nilary-removal.check
@@ -1,4 +1,4 @@
-prefix-unary-nilary-removal.scala:4: error: empty-paren (nilary) prefix unary operator 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`
   def unary_~() : Foo = this
       ^
 1 error

From 8cdf16ef9acc0fe4257169db66ca3a2b6934c542 Mon Sep 17 00:00:00 2001
From: Eugene Yokota <eed3si9n@gmail.com>
Date: Thu, 25 Jun 2020 12:26:34 -0400
Subject: [PATCH 3/3] Print warning for ()(implicit ...) prefix unary operators
 too

---
 .../scala/tools/nsc/ast/parser/Parsers.scala       | 12 +++++++-----
 src/reflect/scala/reflect/internal/Printers.scala  | 14 +++++++++-----
 .../neg/prefix-unary-nilary-deprecation.check      |  9 ++++++---
 .../neg/prefix-unary-nilary-deprecation.scala      |  2 ++
 test/files/neg/prefix-unary-nilary-removal.check   |  7 +++++--
 test/files/neg/prefix-unary-nilary-removal.scala   |  2 ++
 6 files changed, 31 insertions(+), 15 deletions(-)

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index 1961fceeb230..d22279a5def2 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/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
           }
         }
diff --git a/src/reflect/scala/reflect/internal/Printers.scala b/src/reflect/scala/reflect/internal/Printers.scala
index 765278b5745c..b31a8998287f 100644
--- a/src/reflect/scala/reflect/internal/Printers.scala
+++ b/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)
diff --git a/test/files/neg/prefix-unary-nilary-deprecation.check b/test/files/neg/prefix-unary-nilary-deprecation.check
index be1eddcbbb15..ed5ecc0a17ce 100644
--- a/test/files/neg/prefix-unary-nilary-deprecation.check
+++ b/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
diff --git a/test/files/neg/prefix-unary-nilary-deprecation.scala b/test/files/neg/prefix-unary-nilary-deprecation.scala
index 0dad1f65eb6b..3060cda77f97 100644
--- a/test/files/neg/prefix-unary-nilary-deprecation.scala
+++ b/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
diff --git a/test/files/neg/prefix-unary-nilary-removal.check b/test/files/neg/prefix-unary-nilary-removal.check
index ac671f41875c..83b23feca6fb 100644
--- a/test/files/neg/prefix-unary-nilary-removal.check
+++ b/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
diff --git a/test/files/neg/prefix-unary-nilary-removal.scala b/test/files/neg/prefix-unary-nilary-removal.scala
index fde5a1043caa..1b83f9d1df8a 100644
--- a/test/files/neg/prefix-unary-nilary-removal.scala
+++ b/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