Deprecate package objects with inheritance

This deprecates package objects with inheritance, and removes it under -Xsource:2.14.

Ref scala/scala-dev#488
Ref scala/scala-dev#441

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -1881,16 +1881,31 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
 
       if (settings.isScala211  && mdef.symbol == PredefModule)
         ensurePredefParentsAreInSameSourceFile(impl2)
+      if (mdef.name == nme.PACKAGEkw)
+        ensurePackageObjectDoesNotInherit(impl2, mdef.symbol, mdef.pos)
 
       treeCopy.ModuleDef(mdef, typedMods, mdef.name, impl2) setType NoType
     }
 
+    private def ensurePackageObjectDoesNotInherit(template: Template, sym: Symbol, pos: Position): Unit = {
+      def msg(what: String): String = s"package objects with parent classes or traits are $what"
+      def okSymbol(symbol: Symbol): Boolean = (symbol == AnyRefClass) || (symbol == AnyValClass)
+      template.parents match {
+        case Nil => ()
+        case parent :: Nil if okSymbol(parent.symbol) => ()
+        case _ =>
+          if (settings.isScala214) context.error(pos, msg("unsupported"))
+          else context.deprecationWarning(pos, sym, msg("deprecated"), "2.13.0")
+      }
+    }
+
     private def ensurePredefParentsAreInSameSourceFile(template: Template) = {
       val parentSyms = template.parents map (_.symbol) filterNot (_ == AnyRefClass)
       val PredefModuleFile = PredefModule.associatedFile
       if (parentSyms exists (_.associatedFile != PredefModuleFile))
         context.error(template.pos, s"All parents of Predef must be defined in ${PredefModuleFile}.")
     }
+
     /** In order to override this in the TreeCheckers Typer so synthetics aren't re-added
      *  all the time, it is exposed here the module/class typing methods go through it.
      *  ...but it turns out it's also the ideal spot for namer/typer coordination for

test/files/neg/package-inheritance-deprecation.check

@@ -0,0 +1,6 @@
+package-inheritance-deprecation.scala:39: warning: package objects with parent classes or traits are deprecated
+  package object bar extends Duh with Duh2 { }
+                 ^
+error: No warnings can be incurred under -Xfatal-warnings.
+one warning found
+one error found

test/files/neg/package-inheritance-deprecation.flags

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

test/files/neg/package-inheritance-removal.check

@@ -0,0 +1,4 @@
+package-inheritance-removal.scala:39: error: package objects with parent classes or traits are unsupported
+  package object bar extends Duh with Duh2 { }
+                 ^
+one error found

test/files/neg/package-inheritance-removal.flags

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

test/files/neg/package-inheritance-removal.scala

@@ -0,0 +1,62 @@
+// a.scala
+// Fri Jan 13 11:31:47 PST 2012
+
+package foo {
+  package object bar {
+    def duh(n: Long)   = println("long")
+    def duh(n: Double) = println("double")
+
+    def duh2(n: Double) = println("double")
+    def duh2(n: Long)   = println("long")
+  }
+  package bar {
+    object Main {
+      def main(args:Array[String]): Unit = {
+        duh(33L)
+        bip.bar.duh(33L)
+        duh(33d)
+        bip.bar.duh(33d)
+
+        duh2(33L)
+        bip.bar.duh2(33L)
+        duh2(33d)
+        bip.bar.duh2(33d)
+      }
+    }
+  }
+}
+
+package bip {
+  trait Duh {
+    def duh(n: Long)   = println("long")
+    def duh(n: Double) = println("double")
+  }
+  trait Duh2 {
+    def duh2(n: Double) = println("double")
+    def duh2(n: Long)   = println("long")
+  }
+
+  package object bar extends Duh with Duh2 { }
+  package bar {
+    object Main {
+      def main(args:Array[String]): Unit = {
+        duh(33L)
+        bip.bar.duh(33L)
+        duh(33d)
+        bip.bar.duh(33d)
+
+        duh2(33L)
+        bip.bar.duh2(33L)
+        duh2(33d)
+        bip.bar.duh2(33d)
+      }
+    }
+  }
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    foo.bar.Main.main(null)
+    bip.bar.Main.main(null)
+  }
+}

test/files/pos/t5954d.flags

@@ -1 +1 @@
--Xfatal-warnings -Xdev
+-deprecation -Xdev

test/files/run/t1987b.check

@@ -1 +1,4 @@
+pkg2.scala:3: warning: package objects with parent classes or traits are deprecated
+package object xml extends PullIteratees
+               ^
 ok!

test/files/run/t1987b.flags

@@ -0,0 +1 @@
+-deprecation

test/files/run/t5604.check

@@ -1,3 +1,6 @@
+t5604.scala:26: warning: package objects with parent classes or traits are deprecated
+  package object bar extends Duh {
+                 ^
 long
 double
 long

test/files/run/t5604.flags

@@ -0,0 +1 @@
+-deprecation

test/scaladoc/run/package-object.check

@@ -1,3 +1,4 @@
+warning: there was one deprecation warning (since 2.13.0); re-run with -deprecation for details
 List(test.B, test.A, scala.AnyRef, scala.Any)
 List(B, A, AnyRef, Any)
 Some((newSource,10))