Properly type bounds of wildcards in hk positions

Typing a TypeBoundsTree(Empty, Empty) leads to a TypeBoundsTree(Nothing,
Any), if this tree appears in a higher-kinded position, we manually set
its type, but we don't change the upper bound which is still "Any" and
ends up being pickled. To avoid this issue this commit simply replaces
the TypeBoundsTree with a synthetic TypeTree with the proper type and
position, which is all that we need.

Author: smarter

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

@@ -1290,7 +1290,6 @@ class Typer extends Namer
               case _ =>
             }
           if (desugaredArg.isType) {
-            var res = typed(desugaredArg, argPt)
             arg match {
               case TypeBoundsTree(EmptyTree, EmptyTree)
               if tparam.paramInfo.isLambdaSub &&
@@ -1302,10 +1301,10 @@ class Typer extends Namer
                 // type parameter in `C`.
                 // The transform does not apply for patterns, where empty bounds translate to
                 // wildcard identifiers `_` instead.
-                res = res.withType(tparamBounds)
+                TypeTree(tparamBounds).withPos(arg.pos)
               case _ =>
+                typed(desugaredArg, argPt)
             }
-            res
           }
           else desugaredArg.withType(UnspecifiedErrorType)
         }

compiler/test/dotc/pos-test-pickling.blacklist

@@ -5,7 +5,6 @@ i1867.scala
 i3067.scala
 i3149.scala
 i3692.scala
-i3976.scala
 i4125.scala
 implicit-dep.scala
 inline-access-levels