Emit a static forwarder when a bridge overrides a non-deferred method

This seems to match the behavior of Scala 2.13 and more importantly it
will be needed after the next commit that emits mixin forwarders as
bridges, otherwise `extends App` won't work.

Author: smarter

compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala

@@ -388,8 +388,11 @@ trait BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
       }
       debuglog(s"Potentially conflicting names for forwarders: $conflictingNames")
 
-      for (m <- moduleClass.info.membersBasedOnFlags(ExcludedForwarderFlags, Flag_METHOD)) {
-        if (m.isType || m.isDeferred || (m.owner eq ObjectClass) || m.isConstructor || m.isExpanded)
+      for (m0 <- moduleClass.info.membersBasedOnFlags(ExcludedForwarderFlags, Flag_METHOD)) {
+        val m = if (m0.isBridge) m0.nextOverriddenSymbol else m0
+        if (m == NoSymbol)
+          log(s"$m0 is a bridge method that overrides nothing, something went wrong in a previous phase.")
+        else if (m.isType || m.isDeferred || (m.owner eq ObjectClass) || m.isConstructor || m.isExpanded)
           debuglog(s"No forwarder for '$m' from $jclassName to '$moduleClass'")
         else if (conflictingNames(m.name))
           log(s"No forwarder for $m due to conflict with ${linkedClass.info.member(m.name)}")

compiler/src/dotty/tools/backend/jvm/BackendInterface.scala

@@ -537,7 +537,7 @@ abstract class BackendInterface extends BackendInterfaceDefinitions {
     def companionSymbol: Symbol
     def moduleClass: Symbol
     def enclosingClassSym: Symbol
-
+    def nextOverriddenSymbol: Symbol
 
 
     // members

compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -431,7 +431,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   val Flag_METHOD: Flags = Flags.Method.bits
   val ExcludedForwarderFlags: Flags = {
       Flags.Specialized | Flags.Lifted | Flags.Protected | Flags.JavaStatic |
-      Flags.Bridge | Flags.Private | Flags.Macro
+      Flags.Private | Flags.Macro
   }.bits
 
   def isQualifierSafeToElide(qual: Tree): Boolean = tpd.isIdempotentExpr(qual)
@@ -765,6 +765,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
       }
       else sym.enclosingClass(ctx.withPhase(ctx.flattenPhase.prev))
     } //todo is handled specially for JavaDefined symbols in scalac
+    def nextOverriddenSymbol: Symbol = toDenot(sym).nextOverriddenSymbol
 
     // members
     def primaryConstructor: Symbol = toDenot(sym).primaryConstructor

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

@@ -1140,6 +1140,15 @@ object SymDenotations {
       if (!canMatchInheritedSymbols) Iterator.empty
       else overriddenFromType(owner.info)
 
+    /** Equivalent to `allOverriddenSymbols.headOption.getOrElse(NoSymbol)` but more efficient. */
+    final def nextOverriddenSymbol(implicit ctx: Context): Symbol = {
+      val overridden = allOverriddenSymbols
+      if (overridden.hasNext)
+        overridden.next
+      else
+        NoSymbol
+    }
+
     /** Returns all matching symbols defined in parents of the selftype. */
     final def extendedOverriddenSymbols(implicit ctx: Context): Iterator[Symbol] =
       if (!canMatchInheritedSymbols) Iterator.empty