SI-8265 Restore 2.10 variance behavior under -Xsource:2.10

Issue deprecation warning under -Xsource:2.10 so time
travelers can have an authentic deprecation experience before
finding that their unsound code no longer compiles in 2.11.
The relevant ticket to the soundness issue is SI-6566.

6 files changed, 20 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/Global.scala b/src/compiler/scala/tools/nsc/Global.scala
index c5d0c8506a..35eab94333 100644
--- a/src/compiler/scala/tools/nsc/Global.scala
+++ b/src/compiler/scala/tools/nsc/Global.scala
@@ -236,6 +236,7 @@ class Global(var currentSettings: Settings, var reporter: Reporter)
   override def inform(msg: String)      = inform(NoPosition, msg)
   override def globalError(msg: String) = globalError(NoPosition, msg)
   override def warning(msg: String)     = warning(NoPosition, msg)
+  override def deprecationWarning(pos: Position, msg: String) = currentUnit.deprecationWarning(pos, msg)
 
   def globalError(pos: Position, msg: String) = reporter.error(pos, msg)
   def warning(pos: Position, msg: String)     = if (settings.fatalWarnings) globalError(pos, msg) else reporter.warning(pos, msg)
@@ -1236,7 +1237,7 @@ class Global(var currentSettings: Settings, var reporter: Reporter)
     }
 
     private val unitbuf = new SyncedCompilationBuffer
-      
+
     val compiledFiles   = new mutable.HashSet[String]
 
     /** A map from compiled top-level symbols to their source files */
@@ -1491,7 +1492,7 @@ class Global(var currentSettings: Settings, var reporter: Reporter)
        made to the underlying structure.
      */
     def units: Iterator[CompilationUnit] = unitbuf.iterator
-    
+
     def registerPickle(sym: Symbol): Unit = ()
 
     /** does this run compile given class, module, or case factory? */
diff --git a/src/reflect/scala/reflect/internal/SymbolTable.scala b/src/reflect/scala/reflect/internal/SymbolTable.scala
index e50c65c9ca..c76dedbff4 100644
--- a/src/reflect/scala/reflect/internal/SymbolTable.scala
+++ b/src/reflect/scala/reflect/internal/SymbolTable.scala
@@ -51,6 +51,7 @@ abstract class SymbolTable extends macros.Universe
   val gen = new InternalTreeGen { val global: SymbolTable.this.type = SymbolTable.this }
 
   def log(msg: => AnyRef): Unit
+  def deprecationWarning(pos: Position, msg: String): Unit = warning(msg)
   def warning(msg: String): Unit     = Console.err.println(msg)
   def inform(msg: String): Unit      = Console.err.println(msg)
   def globalError(msg: String): Unit = abort(msg)
diff --git a/src/reflect/scala/reflect/internal/Variances.scala b/src/reflect/scala/reflect/internal/Variances.scala
index 3bcfed7d34..cfe2ad8b87 100644
--- a/src/reflect/scala/reflect/internal/Variances.scala
+++ b/src/reflect/scala/reflect/internal/Variances.scala
@@ -75,7 +75,14 @@ trait Variances {
         def nextVariance(sym: Symbol, v: Variance): Variance = (
           if (shouldFlip(sym, tvar)) v.flip
           else if (isLocalOnly(sym)) Bivariant
-          else if (sym.isAliasType) Invariant
+          else if (sym.isAliasType) (
+            // Unsound pre-2.11 behavior preserved under -Xsource:2.10
+            if (settings.isScala211 || sym.isOverridingSymbol) Invariant
+            else {
+              deprecationWarning(sym.pos, s"Construct depends on unsound variance analysis and will not compile in scala 2.11 and beyond")
+              Bivariant
+            }
+          )
           else v
         )
         def loop(sym: Symbol, v: Variance): Variance = (
diff --git a/test/files/neg/t8265.check b/test/files/neg/t8265.check
new file mode 100644
index 0000000000..7b1db1c4e5
--- /dev/null
+++ b/test/files/neg/t8265.check
@@ -0,0 +1,6 @@
+t8265.scala:1: warning: Construct depends on unsound variance analysis and will not compile in scala 2.11 and beyond
+class Foo[+CC[X]] { type Coll = CC[_] }
+                         ^
+error: No warnings can be incurred under -Xfatal-warnings.
+one warning found
+one error found
diff --git a/test/files/neg/t8265.flags b/test/files/neg/t8265.flags
new file mode 100644
index 0000000000..9d7ba7abd8
--- /dev/null
+++ b/test/files/neg/t8265.flags
@@ -0,0 +1 @@
+-Xsource:2.10 -deprecation -language:higherKinds -Xfatal-warnings
diff --git a/test/files/neg/t8265.scala b/test/files/neg/t8265.scala
new file mode 100644
index 0000000000..a215903ebc
--- /dev/null
+++ b/test/files/neg/t8265.scala
@@ -0,0 +1 @@
+class Foo[+CC[X]] { type Coll = CC[_] }