Added a HIDDEN flag.

[Still not a fan of the massive overlap between GenJVM and GenASM. I do
not think such duplication should be allowed into master at all, neither
in experimental backends nor anywhere else. Putting us in the position
that we have to actually switch to stop incurring the duplication is
engineering-by-prayer.]

The SYNTHETIC flag was long ago discovered to be inadequately specific
to allow for emitting ACC_SYNTHETIC in bytecode. In this commit is born
a HIDDEN flag, which signals the flagged symbol as implementation detail
which should not be considered during typechecking, and which will
receive ACC_SYNTHETIC during code generation.

Unsure what should be hidden, I conservatively marked a few things which
seem safely hidable.

 - $outer fields and accessors
 - classes whose classfile has the jvm Synthetic attribute
   (not to be confused with the SYNTHETIC flag)

I leave additional choices to those who have a better idea how
this will materialize (i.e. IDE guys.)

It is easy to selectively introduce this flag; but the SYNTHETIC flag
is set or checked for in so many places, it is very difficult to alter
the logic around it (either by setting it less, or checking for HIDDEN
only) with much confidence. So right now HIDDEN is only used to help
ACC_SYNTHETIC make it into bytecode - it is only set in conjunction with
SYNTHETIC, and it doesn't help anyone hide from the typechecker.

Review by @dragos, @odersky.

5 files changed, 9 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala b/src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala
index 5ab8a3d751..da06ea3791 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala
@@ -345,6 +345,7 @@ abstract class GenASM extends SubComponent with BytecodeWriters {
       if (finalFlag && !sym.hasAbstractFlag) ACC_FINAL else 0,

       if (sym.isStaticMember) ACC_STATIC else 0,

       if (sym.isBridge) ACC_BRIDGE | ACC_SYNTHETIC else 0,

+      if (sym.isHidden) ACC_SYNTHETIC else 0,

       if (sym.isClass && !sym.isInterface) ACC_SUPER else 0,

       if (sym.isVarargsMethod) ACC_VARARGS else 0,

       if (sym.hasFlag(Flags.SYNCHRONIZED)) ACC_SYNCHRONIZED else 0

@@ -889,7 +890,7 @@ abstract class GenASM extends SubComponent with BytecodeWriters {
       // without it.  This is particularly bad because the availability of

       // generic information could disappear as a consequence of a seemingly

       // unrelated change.

-         sym.isSynthetic

+         sym.isHidden

       || sym.isLiftedMethod

       || sym.isBridge

       || (sym.ownerChain exists (_.isImplClass))

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/GenJVM.scala b/src/compiler/scala/tools/nsc/backend/jvm/GenJVM.scala
index ad054015ef..bae59ffaf8 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/GenJVM.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/GenJVM.scala
@@ -735,7 +735,7 @@ abstract class GenJVM extends SubComponent with GenJVMUtil with GenAndroid with
       // without it.  This is particularly bad because the availability of
       // generic information could disappear as a consequence of a seemingly
       // unrelated change.
-         sym.isSynthetic
+         sym.isHidden
       || sym.isLiftedMethod
       || sym.isBridge
       || (sym.ownerChain exists (_.isImplClass))
@@ -1980,6 +1980,7 @@ abstract class GenJVM extends SubComponent with GenJVMUtil with GenAndroid with
       if (finalFlag && !sym.hasAbstractFlag) ACC_FINAL else 0,
       if (sym.isStaticMember) ACC_STATIC else 0,
       if (sym.isBridge) ACC_BRIDGE | ACC_SYNTHETIC else 0,
+      if (sym.isHidden) ACC_SYNTHETIC else 0,
       if (sym.isClass && !sym.isInterface) ACC_SUPER else 0,
       if (sym.isVarargsMethod) ACC_VARARGS else 0,
       if (sym.hasFlag(Flags.SYNCHRONIZED)) JAVA_ACC_SYNCHRONIZED else 0
diff --git a/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala b/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
index 65b0ff1e6d..e6499c05a6 100644
--- a/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
+++ b/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
@@ -862,7 +862,7 @@ abstract class ClassfileParser {
           }
           else in.skip(attrLen)
         case tpnme.SyntheticATTR =>
-          sym.setFlag(SYNTHETIC)
+          sym.setFlag(SYNTHETIC | HIDDEN)
           in.skip(attrLen)
         case tpnme.BridgeATTR =>
           sym.setFlag(BRIDGE)
diff --git a/src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala b/src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala
index 1b8513373d..ab7bbc591b 100644
--- a/src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala
+++ b/src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala
@@ -92,7 +92,7 @@ abstract class ExplicitOuter extends InfoTransform
     else findOrElse(clazz.info.decls)(_.outerSource == clazz)(NoSymbol)
   }
   def newOuterAccessor(clazz: Symbol) = {
-    val accFlags = SYNTHETIC | METHOD | STABLE | ( if (clazz.isTrait) DEFERRED else 0 )
+    val accFlags = SYNTHETIC | HIDDEN | METHOD | STABLE | ( if (clazz.isTrait) DEFERRED else 0 )
     val sym      = clazz.newMethod(nme.OUTER, clazz.pos, accFlags)
     val restpe   = if (clazz.isTrait) clazz.outerClass.tpe else clazz.outerClass.thisType
 
@@ -101,7 +101,7 @@ abstract class ExplicitOuter extends InfoTransform
     sym setInfo MethodType(Nil, restpe)
   }
   def newOuterField(clazz: Symbol) = {
-    val accFlags = SYNTHETIC | PARAMACCESSOR | ( if (clazz.isEffectivelyFinal) PrivateLocal else PROTECTED )
+    val accFlags = SYNTHETIC | HIDDEN | PARAMACCESSOR | ( if (clazz.isEffectivelyFinal) PrivateLocal else PROTECTED )
     val sym      = clazz.newValue(nme.OUTER_LOCAL, clazz.pos, accFlags)
 
     sym setInfo clazz.outerClass.thisType
diff --git a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
index 53c2d16928..53843adea9 100644
--- a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
@@ -8,7 +8,7 @@ package scala.tools.nsc
 package typechecker
 
 import symtab._
-import Flags.{MUTABLE, METHOD, LABEL, SYNTHETIC}
+import Flags.{MUTABLE, METHOD, LABEL, SYNTHETIC, HIDDEN}
 import language.postfixOps
 import scala.tools.nsc.transform.TypingTransformers
 import scala.tools.nsc.transform.Transform
@@ -949,7 +949,7 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
 
           // ExplicitOuter replaces `Select(q, outerSym) OBJ_EQ expectedPrefix` by `Select(q, outerAccessor(outerSym.owner)) OBJ_EQ expectedPrefix`
           // if there's an outer accessor, otherwise the condition becomes `true` -- TODO: can we improve needsOuterTest so there's always an outerAccessor?
-          val outer = expectedTp.typeSymbol.newMethod(vpmName.outer) setInfo expectedTp.prefix setFlag SYNTHETIC
+          val outer = expectedTp.typeSymbol.newMethod(vpmName.outer) setInfo expectedTp.prefix setFlag SYNTHETIC | HIDDEN
 
           (Select(codegen._asInstanceOf(testedBinder, expectedTp), outer)) OBJ_EQ expectedOuter
         }