avoidCaptures: handle local types, not just terms

Not that the added test case still infers `Local` for the type of `a`
because avoidance doesn't handle type variables correctly and because
the nesting level checks are too coarse, this doesn't lead to an error
because the check for forward references in TreePickler is currently
disabled. All these issues are fixed in later commits of this PR.

Author: smarter

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

@@ -4715,10 +4715,10 @@ object Types {
     private def avoidCaptures(tp: Type)(using Context): Type =
       val problemSyms = new TypeAccumulator[Set[Symbol]]:
         def apply(syms: Set[Symbol], t: Type): Set[Symbol] = t match
-          case ref @ TermRef(NoPrefix, _)
+          case ref: NamedType
           // AVOIDANCE TODO: Are there other problematic kinds of references?
           // Our current tests only give us these, but we might need to generalize this.
-          if ref.symbol.maybeOwner.nestingLevel > nestingLevel =>
+          if (ref.prefix eq NoPrefix) && ref.symbol.maybeOwner.nestingLevel > nestingLevel =>
             syms + ref.symbol
           case _ =>
             foldOver(syms, t)

tests/pos/i8900a.scala

@@ -0,0 +1,11 @@
+class Inv[T](val elem: T)
+object Test {
+  def unwrap[Outer](inv: Inv[Outer]): Outer = inv.elem
+  def wrap[Inner](i: Inner): Inv[Inner] = new Inv(i)
+
+  val a = unwrap({
+    class Local
+    val local = new Local
+    wrap(local)
+  })
+}