Class symbols can't be contravariant.

During development of the fix for SI-6666, I encountered:

    % test/files/pos/t4842.scala
    test/files/pos/t4842.scala:10: error: contravariant class Bar occurs in covariant position in type ()this.Bar of constructor Bar
         this(new { class Bar { println(Bar.this); new { println(Bar.this) } }; new Bar } ) // okay

I had incorrectly set the INCONSTRUCTOR flag on the class symbol
`Bar`. (It isn't directly in the self constructor call, as it
is nested an intervening anonymous class.)

But, this flag shares a slot with CONTRAVARIANT, and the variance
validation intepreted it as such.

ClassSymbol already has this code to resolve the ambiguous
flags for display purposes:

    override def resolveOverloadedFlag(flag: Long) = flag match {
      case INCONSTRUCTOR => "<inconstructor>" // INCONSTRUCTOR / CONTRAVARIANT / LABEL
      case EXISTENTIAL   => "<existential>"   // EXISTENTIAL / MIXEDIN
      case IMPLCLASS     => "<implclass>"     // IMPLCLASS / PRESUPER
      case _             => super.resolveOverloadedFlag(flag)
    }

This commit overrides `isContravariant` to reflect the same logic.

Author: retronym

src/compiler/scala/tools/nsc/interpreter/IMain.scala

@@ -262,7 +262,9 @@ class IMain(initialSettings: Settings, protected val out: JPrintWriter) extends
   protected def newCompiler(settings: Settings, reporter: Reporter): ReplGlobal = {
     settings.outputDirs setSingleOutput virtualDirectory
     settings.exposeEmptyPackage.value = true
-    new Global(settings, reporter) with ReplGlobal
+    new Global(settings, reporter) with ReplGlobal {
+      override def toString: String = "<global>"
+    }
   }
 
   /** Parent classloader.  Overridable. */

src/reflect/scala/reflect/internal/Symbols.scala

@@ -2867,6 +2867,7 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     final override def isNonClassType = false
     final override def isAbstractType = false
     final override def isAliasType = false
+    final override def isContravariant = false
 
     override def isAbstractClass           = this hasFlag ABSTRACT
     override def isCaseClass               = this hasFlag CASE

test/files/run/class-symbol-contravariant.check

@@ -0,0 +1,36 @@
+Type in expressions to have them evaluated.
+Type :help for more information.
+
+scala> :power
+** Power User mode enabled - BEEP WHIR GYVE **
+** :phase has been set to 'typer'.          **
+** scala.tools.nsc._ has been imported      **
+** global._, definitions._ also imported    **
+** Try  :help, :vals, power.<tab>           **
+
+scala> val u = rootMirror.universe
+u: $r.intp.global.type = <global>
+
+scala> import u._, scala.reflect.internal.Flags
+import u._
+import scala.reflect.internal.Flags
+
+scala> class C
+defined class C
+
+scala> val sym = u.typeOf[C].typeSymbol
+sym: u.Symbol = class C
+
+scala> sym.isContravariant
+res0: Boolean = false
+
+scala> sym setFlag Flags.INCONSTRUCTOR
+res1: sym.type = class C
+
+scala> sym.isClassLocalToConstructor
+res2: Boolean = true
+
+scala> sym.isContravariant // was true
+res3: Boolean = false
+
+scala> 

test/files/run/class-symbol-contravariant.scala

@@ -0,0 +1,15 @@
+import scala.tools.partest.ReplTest
+
+object Test extends ReplTest {
+  override def code = """
+  |:power
+  |val u = rootMirror.universe
+  |import u._, scala.reflect.internal.Flags
+  |class C
+  |val sym = u.typeOf[C].typeSymbol
+  |sym.isContravariant
+  |sym setFlag Flags.INCONSTRUCTOR
+  |sym.isClassLocalToConstructor
+  |sym.isContravariant // was true
+  |""".stripMargin.trim
+}