Do not create symbols for classfile artifacts

These definitions are not supposed to be called from user code, so
creating symbols for them is unnecessary.
In particular, this fixes various issues with bridges:
- Fix #6152: We require `override` to implement bridged abstract method
- Fix #8435: Generic signature is missing for bridged method

(This PR is only possible thanks to #8499, without this we would use
non-bridge symbols as receivers and get IllegalAccessErrors in various
cases).

Note that this is slightly different from what Scala 2 does:
`Type#member` in scalac excludes symbols with the Bridge flag set by
default, we can't copy that behavior without more changes on our end
since member filtering works differently in both compiler: in Scala 2 if
a symbol with an excluded flag overrides a symbol without the flag, the
overridden symbol is returned, whereas no symbol is returned in Dotty.
In any case, I think that simply not creating the symbols is a more
systematic solution, it also makes separate compilation of .java and
.scala files work more like joint compilation since under joint
compilation we never see the synthetic members generated by javac
anyway, this reduces the risk of bugs which appear only under one
kind of compilation and not the other.

Author: smarter

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -567,6 +567,7 @@ object Flags {
   val TermParamOrAccessor: FlagSet           = Param | ParamAccessor
   val PrivateParamAccessor: FlagSet          = ParamAccessor | Private
   val PrivateOrSynthetic: FlagSet            = Private | Synthetic
+  val PrivateOrArtifact: FlagSet             = Private | Artifact
   val ClassTypeParam: FlagSet                = Private | TypeParam
   val Scala2Trait: FlagSet                   = Scala2x | Trait
   val SyntheticArtifact: FlagSet             = Synthetic | Artifact

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -226,7 +226,7 @@ class ClassfileParser(
       if (method) Flags.Method | methodTranslation.flags(jflags)
       else fieldTranslation.flags(jflags)
     val name = pool.getName(in.nextChar)
-    if (!sflags.is(Flags.Private) || name == nme.CONSTRUCTOR) {
+    if (!sflags.isOneOf(Flags.PrivateOrArtifact) || name == nme.CONSTRUCTOR) {
       val member = ctx.newSymbol(
         getOwner(jflags), name, sflags, memberCompleter,
         getPrivateWithin(jflags), coord = start)

tests/pos-java-interop-separate/i6152/A_1.java

@@ -0,0 +1,8 @@
+public abstract class A_1 {
+  public abstract Object f();
+
+  public static abstract class B extends A_1 {
+    @Override
+    public abstract String f();
+  }
+}

tests/pos-java-interop-separate/i6152/C_2.scala

@@ -0,0 +1,3 @@
+class C extends A_1.B {
+  def f() = ""
+}

tests/pos-java-interop-separate/i8435/Builder_1.java

@@ -0,0 +1,4 @@
+class BuilderBase<A> {
+  public scala.Option<A> build() { return null; }
+}
+public class Builder_1<A> extends BuilderBase<A> { }

tests/pos-java-interop-separate/i8435/Test_2.scala

@@ -0,0 +1,4 @@
+object Test {
+  val builder: Builder_1[Int] = ???
+  builder.build(): Option[Int]
+}