Revert "Remove now-unnecessary avoid when typing closures"

More precisely, put it under a Config flag which is off by default.

This reverts commit 629006b.

Author: odersky

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

@@ -1672,7 +1672,23 @@ class Namer { typer: Typer =>
             // This case applies if the closure result type contains uninstantiated
             // type variables. In this case, constrain the closure result from below
             // by the parameter-capture-avoiding type of the body.
-            typedAheadExpr(mdef.rhs, tpt.tpe).tpe
+            val rhsType = typedAheadExpr(mdef.rhs, tpt.tpe).tpe
+
+            // The following part is important since otherwise we might instantiate
+            // the closure result type with a plain functon type that refers
+            // to local parameters. An example where this happens in `dependent-closures.scala`
+            // If the code after `val rhsType` is commented out, this file fails pickling tests.
+            // AVOIDANCE TODO: Follow up why this happens, and whether there
+            // are better ways to achieve this. It would be good if we could get rid of this code.
+            // It seems at least partially redundant with the nesting level checking on TypeVar
+            // instantiation.
+            if !Config.checkLevels then
+              val hygienicType = TypeOps.avoid(rhsType, termParamss.flatten)
+              if (!hygienicType.isValueType || !(hygienicType <:< tpt.tpe))
+                report.error(i"return type ${tpt.tpe} of lambda cannot be made hygienic;\n" +
+                  i"it is not a supertype of the hygienic type $hygienicType", mdef.srcPos)
+            //println(i"lifting $rhsType over $termParamss -> $hygienicType = ${tpt.tpe}")
+            //println(TypeComparer.explained { implicit ctx => hygienicType <:< tpt.tpe })
           case _ =>
         }
         WildcardType