Fix lack of type avoidance in argument lifting

We need to manually increase the nestingLevel of symbols created by
EtaExpansion#lift to compensate for the fact that they will end up in a block.

Fixes scala#15174.

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

@@ -512,14 +512,15 @@ object Symbols {
 // ---- Symbol creation methods ----------------------------------
 
   /** Create a symbol from its fields (info may be lazy) */
-  def newSymbol[N <: Name](
+  def newSymbol[N <: Name](using Context)(
       owner: Symbol,
       name: N,
       flags: FlagSet,
       info: Type,
       privateWithin: Symbol = NoSymbol,
-      coord: Coord = NoCoord)(using Context): Symbol { type ThisName = N } = {
-    val sym = new Symbol(coord, ctx.base.nextSymId, ctx.nestingLevel).asInstanceOf[Symbol { type ThisName = N }]
+      coord: Coord = NoCoord,
+      nestingLevel: Int = ctx.nestingLevel): Symbol { type ThisName = N } = {
+    val sym = new Symbol(coord, ctx.base.nextSymId, nestingLevel).asInstanceOf[Symbol { type ThisName = N }]
     val denot = SymDenotation(sym, owner, name, flags, info, privateWithin)
     sym.denot = denot
     sym

compiler/src/dotty/tools/dotc/typer/EtaExpansion.scala

@@ -48,7 +48,10 @@ abstract class Lifter {
       // don't instantiate here, as the type params could be further constrained, see tests/pos/pickleinf.scala
       var liftedType = expr.tpe.widen.deskolemized
       if (liftedFlags.is(Method)) liftedType = ExprType(liftedType)
-      val lifted = newSymbol(ctx.owner, name, liftedFlags | Synthetic, liftedType, coord = spanCoord(expr.span))
+      val lifted = newSymbol(ctx.owner, name, liftedFlags | Synthetic, liftedType, coord = spanCoord(expr.span),
+        // Lifted definitions will be added to a local block, so they need to be
+        // at a higher nesting level to prevent leaks. See tests/pos/i15174.scala
+        nestingLevel = ctx.nestingLevel + 1)
       defs += liftedDef(lifted, expr)
         .withSpan(expr.span)
         .changeNonLocalOwners(lifted)

tests/pos/i15174.scala

@@ -0,0 +1,34 @@
+trait Error
+sealed abstract class Codec[A] {
+  type AvroType
+  def encode(a: A): Either[Error, AvroType]
+  // def decode(value: Any): Either[Error, A]
+}
+
+object Codec {
+  type Aux[AvroType0, A] = Codec[A] {
+    type AvroType = AvroType0
+  }
+
+  final def instance[AvroType0, A](
+      encode: A => Either[Error, AvroType0],
+      // decode: Any => Either[Error, A]
+  ): Codec.Aux[AvroType0, A]  = ???
+
+  implicit final def option[A](implicit codec: Codec[A]): Codec[Option[A]] = ???
+  given Codec.Aux[Int, Int] = ???
+}
+
+
+@main def test() = {
+  implicit val codec: Codec[Option[Int]] =
+    Codec.instance(
+      Codec.option[Int].encode
+        // expands to:
+        // {
+        //   val a: Codec[Option[Int]] = Codec.option[Int](Codec.given_Aux_Int_Int)
+        //   a.encode
+        // },
+      // Codec.option[Int].decode
+    )
+}