Fix Lucre

The test neg/i12169.scala explains what went wrong and how to fix it. Before this PR Lurcre compiled,
since the wildcard was translated to an abstract type that was not entered in scope and that therefore
escaped avoidance.

Author: odersky

community-build/community-projects/Lucre

@@ -0,0 +1,37 @@
+object Var:
+  class Expanded[T <: Txn[T], B] extends Form[T]
+
+trait Txn[T <: Txn[T]]
+
+trait Form[T]
+
+class TT extends Txn[TT]
+
+private final class FlatVarCellView[T <: Txn[T], B](
+  firstVr  : Option[Var.Expanded[TT, B]]
+)
+
+def Test =
+  val opt: Option[Form[TT]] = ???
+  val firstVr = opt match
+    case Some(ex: Var.Expanded[TT, _]) => Some(ex)
+    case _                            => None
+  new FlatVarCellView(firstVr)  // error
+   //                      Found:    (firstVr : Option[Var.Expanded[TT, ?]])
+   //                      Required: Option[Var.Expanded[TT, B]]
+   //
+   //                      where:    B is a type variable
+   //
+   // Note that we cannot do capture conversion since the `?` does not appear as an argument
+   // of the a type. It's dubious whether capture conversion for more deeply nested types
+   // would be sound.
+
+  // Remedy:
+  opt match
+    case Some(ex: Var.Expanded[TT, _]) => new FlatVarCellView(Some(ex))
+      // here, we instantiate `B` with the unnamed second parameter of `Var.Expanded`
+    case _                             => new FlatVarCellView(None)
+  opt match
+    case Some(ex: Var.Expanded[TT, t]) => new FlatVarCellView[TT, t](Some(ex))
+      // the same as above, spelt out
+    case _                             => new FlatVarCellView(None)