Unmapped type from capture checking with levels

[EugeneFlesselle]

Adding MapView to the capture checking standard library tests (here) works fine before using levels for capture escape checking.

However, on the main branch after #18463, we get the following error:

-- [E007] Type Mismatch Error: tests/pos-special/stdlib/collection/MapView.scala:45:99 ---------------------------------
45 |  override def filterKeys(p: K => Boolean): MapView[K, V]^{this, p} = new MapView.FilterKeys(this, p)
   |                                                                                                   ^
   |                                                                                    Found:    K ->{p} Boolean
   |                                                                                    Required: box K^? => Boolean
   |--------------------------------------------------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   |
   | Tree: p
   | I tried to show that
   |   K ->{p} Boolean
   | conforms to
   |   box K^? => Boolean
   | but the comparison trace ended with `false`:
   |
   |   ==> K ->{p} Boolean  <:  box K^? => Boolean
   |     ==> subcaptures {p} <:< {cap} 
   |     <== subcaptures {p} <:< {cap}  = {cap}
   |     ==> subcaptures {p} <:< {cap} 
   |     <== subcaptures {p} <:< {cap}  = {cap}
   |   <== K ->{p} Boolean  <:  box K^? => Boolean = false
   |
   | The tests were made under the empty constraint
    --------------------------------------------------------------------------------------------------------------------

I still expected it to conform, but I am not familiar with the new changes using levels.

[EugeneFlesselle]

Explicitly declaring the self type of MapView also introduces the following error (not present before levels):

-- [E007] Type Mismatch Error: tests/pos-special/stdlib/collection/MapView.scala:46:93 ---------------------------------
46 |  override def filterKeys(p: K => Boolean): MapView[K, V]^{this, p} = new MapView.FilterKeys(this, p)
   |                                                                                             ^^^^
   |            Found:    (MapView.this : scala.collection.MapView[K, V]^{cap[MapView]})
   |            Required: scala.collection.MapOps[box K^?, box V^?, scala.collection.MapView.SomeIterableConstr, ?]^
   |--------------------------------------------------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   |
   | Tree: this
   | I tried to show that
   |   (MapView.this : scala.collection.MapView[K, V]^{cap[MapView]})
   | conforms to
   |   scala.collection.MapOps[box K^?, box V^?, scala.collection.MapView.SomeIterableConstr, ?]^
   | but the comparison trace ended with `false`:
   |
   |   ==> (MapView.this : scala.collection.MapView[K, V]^{cap[MapView]})  <:  scala.collection.MapOps[box K^?, box V^?, scala.collection.MapView.SomeIterableConstr, ?]^
   |     ==> subcaptures {MapView.this} <:< {cap} 
   |     <== subcaptures {MapView.this} <:< {cap}  = {cap}
   |     ==> scala.collection.MapView[K, V]^{MapView.this}  <:  scala.collection.MapOps[box K^?, box V^?, scala.collection.MapView.SomeIterableConstr, ?]^ (left is approximated)
   |       ==> subcaptures {MapView.this} <:< {cap} 
   |       <== subcaptures {MapView.this} <:< {cap}  = {cap}
   |       ==> subcaptures {MapView.this} <:< {cap} 
   |       <== subcaptures {MapView.this} <:< {cap}  = {cap}
   |     <== scala.collection.MapView[K, V]^{MapView.this}  <:  scala.collection.MapOps[box K^?, box V^?, scala.collection.MapView.SomeIterableConstr, ?]^ (left is approximated) = false
   |   <== (MapView.this : scala.collection.MapView[K, V]^{cap[MapView]})  <:  scala.collection.MapOps[box K^?, box V^?, scala.collection.MapView.SomeIterableConstr, ?]^ = false
   |
   | The tests were made under the empty constraint
    --------------------------------------------------------------------------------------------------------------------