Deprecate procedure syntax unconditionally

Procedure syntax was deprecated under -Xfuture flag in scala#3076.
This deprecates it unconditionally, and drops it under -Xsource:2.14.

See scala/bug#7605

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

@@ -2727,18 +2727,20 @@ self =>
         val vparamss = paramClauses(name, contextBoundBuf.toList, ofCaseClass = false)
         newLineOptWhenFollowedBy(LBRACE)
         var restype = fromWithinReturnType(typedOpt())
+        def msg(what: String, instead: String) =
+          s"procedure syntax is $what: instead, add `$instead` to explicitly declare `$name`'s return type"
         val rhs =
           if (isStatSep || in.token == RBRACE) {
             if (restype.isEmpty) {
-              if (settings.future)
-                deprecationWarning(in.lastOffset, s"Procedure syntax is deprecated. Convert procedure `$name` to method by adding `: Unit`.", "2.12.0")
+              if (settings.isScala214) syntaxError(in.lastOffset, msg("unsupported", ": Unit"))
+              else deprecationWarning(in.lastOffset, msg("deprecated", ": Unit"), "2.13.0")
               restype = scalaUnitConstr
             }
             newmods |= Flags.DEFERRED
             EmptyTree
           } else if (restype.isEmpty && in.token == LBRACE) {
-            if (settings.future)
-              deprecationWarning(in.offset, s"Procedure syntax is deprecated. Convert procedure `$name` to method by adding `: Unit =`.", "2.12.0")
+            if (settings.isScala214) syntaxError(in.lastOffset, msg("unsupported", ": Unit ="))
+            else deprecationWarning(in.offset, msg("deprecated", ": Unit ="), "2.13.0")
             restype = scalaUnitConstr
             blockExpr()
           } else {

test/files/neg/procedure-deprecation.check

@@ -0,0 +1,15 @@
+procedure-deprecation.scala:2: warning: procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `bar`'s return type
+  def bar {}
+          ^
+procedure-deprecation.scala:3: warning: procedure syntax is deprecated: instead, add `: Unit` to explicitly declare `baz`'s return type
+  def baz
+         ^
+procedure-deprecation.scala:4: warning: procedure syntax is deprecated: instead, add `: Unit` to explicitly declare `boo`'s return type
+  def boo(i: Int, l: Long)
+                          ^
+procedure-deprecation.scala:5: warning: procedure syntax is deprecated: instead, add `: Unit =` to explicitly declare `boz`'s return type
+  def boz(i: Int, l: Long) {}
+                           ^
+error: No warnings can be incurred under -Xfatal-warnings.
+four warnings found
+one error found

test/files/neg/procedure-deprecation.flags

@@ -0,0 +1 @@
+-deprecation -Xfatal-warnings

test/files/neg/procedure-removal.check

@@ -0,0 +1,13 @@
+procedure-removal.scala:2: error: procedure syntax is unsupported: instead, add `: Unit =` to explicitly declare `bar`'s return type
+  def bar {}
+         ^
+procedure-removal.scala:3: error: procedure syntax is unsupported: instead, add `: Unit` to explicitly declare `baz`'s return type
+  def baz
+         ^
+procedure-removal.scala:4: error: procedure syntax is unsupported: instead, add `: Unit` to explicitly declare `boo`'s return type
+  def boo(i: Int, l: Long)
+                          ^
+procedure-removal.scala:5: error: procedure syntax is unsupported: instead, add `: Unit =` to explicitly declare `boz`'s return type
+  def boz(i: Int, l: Long) {}
+                          ^
+four errors found

test/files/neg/procedure-removal.flags

@@ -0,0 +1 @@
+-Xsource:2.14

test/files/neg/procedure-removal.scala

@@ -0,0 +1,8 @@
+abstract class Foo {
+  def bar {}
+  def baz
+  def boo(i: Int, l: Long)
+  def boz(i: Int, l: Long) {}
+  def this(i: Int) { this() } // Don't complain here!
+  def foz: Unit               // Don't complain here!
+}