clear all flags when resetting a symbol

this change is a bit scary because it changes code that's not been
changed in 11 years
scala@7fa7c93#diff-d5789e5ae5061197d782d08324b260dbL214

Author: lrytz

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

@@ -725,7 +725,7 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
 
     def setFlag(mask: Long): this.type   = { _rawflags |= mask ; this }
     def resetFlag(mask: Long): this.type = { _rawflags &= ~mask ; this }
-    def resetFlags() { rawflags &= TopLevelCreationFlags }
+    def resetFlags() { rawflags = 0 }
 
     /** Default implementation calls the generic string function, which
      *  will print overloaded flags as <flag1/flag2/flag3>.  Subclasses

test/junit/scala/tools/nsc/backend/jvm/DirectCompileTest.scala

@@ -88,4 +88,15 @@ class DirectCompileTest extends BytecodeTesting {
   def compileErroneous(): Unit = {
     compileToBytes("class C { def f: String = 1 }", allowMessage = _.msg contains "type mismatch")
   }
+
+  @Test
+  def residentRedefineFinalFlag(): Unit = {
+    val compiler = newCompiler()
+    val a = "final class C { def c1 = 0 }"
+    // for re-defined class symbols (C), the compiler did not clear the `final` flag.
+    // so compiling `D` would give an error `illegal inheritance from final class C`.
+    val b = "class C; class D extends C"
+    compiler.compileToBytes(a)
+    compiler.compileToBytes(b)
+  }
 }