Fix #3476: Add Artifact flag to closures

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -805,7 +805,7 @@ object desugar {
    *  If `inlineable` is true, tag $anonfun with an @inline annotation.
    */
   def makeClosure(params: List[ValDef], body: Tree, tpt: Tree = TypeTree(), inlineable: Boolean)(implicit ctx: Context) = {
-    var mods = synthetic
+    var mods = synthetic | Artifact
     if (inlineable) mods |= Inline
     Block(
       DefDef(nme.ANON_FUN, Nil, params :: Nil, tpt, body).withMods(mods),

compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala

@@ -12,6 +12,7 @@ import core.TypeErasure.erasure
 import core.Types._
 import core.classfile.ClassfileConstants
 import ast.Trees._
+import SymUtils._
 import TypeUtils._
 import java.lang.StringBuilder
 
@@ -500,7 +501,8 @@ object GenericSignatures {
           case PolyType(_, _) =>
             true
           case ClassInfo(_, _, parents, _, _) =>
-            foldOver(tp.typeParams.nonEmpty, parents)
+            // Local classes don't need a signature
+            !tp.classSymbol.isLocal && foldOver(tp.typeParams.nonEmpty, parents)
           case AnnotatedType(tpe, _) =>
             foldOver(x, tpe)
           case proxy: TypeProxy =>

compiler/src/dotty/tools/dotc/transform/LambdaLift.scala

@@ -341,7 +341,7 @@ object LambdaLift {
         local.copySymDenotation(
           owner = newOwner,
           name = newName(local),
-          initFlags = local.flags &~ Module &~ Final | Private | maybeStatic,
+          initFlags = local.flags &~ Module &~ Final | Private | Lifted | maybeStatic,
             // drop Module because class is no longer a singleton in the lifted context.
           info = liftedInfo(local)).installAfter(thisPhase)
       }

tests/generic-java-signatures/i3476.check

@@ -0,0 +1 @@
+OK

tests/generic-java-signatures/i3476.scala

@@ -0,0 +1,14 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val members = classOf[Foo].getDeclaredMethods.map(_.toGenericString)
+    assert(members.forall(_ != "<java.lang.NullPointerException>"), members.mkString(", "))
+    println("OK")
+  }
+}
+class Foo {
+  def m[T](x: T): Int => T = {
+    def bar(y: Int): T = x
+    bar _
+  }
+}
+