SI-5022 Retain precise existentials through pattern matching

From the dawn of scalac's existentials, the typer widens
existentials pt-s by substituting wildcard types in places
of existential quantifiers.

In this example:

    class ForSomeVsUnapply {
      def test {
        def makeWrap: Wrap = ???
        def useRep[e](rep: (e, X[e])) = ()

        val rep = makeWrap match {
          case Wrap(r) => r
        };

        useRep(rep) // error
      }
    }

the type of `r` is the result of typechecking:

    Apply(
     fun = TypeTree(
       tpe = (rep#12037: (e#12038, X#7041[e#12038]) forSome { type e#12038 })
     args = Bind(r @ _) :: Nil
    }

This descends to type the `Bind` with:

    pt = (e#12038, X#7041[e#12038]) forSome { type e#12038 }

`dropExistential` clobbers that type to `Tuple2#1540[?, X#7041[?]]`,
which doesn't express any relationship between the two instances
of the wildcard type. `typedIdent` sort of reverses this with a call
to `makeFullyDefined`, but only ends up with:

    pt = (Any#3330, X#7041[_1#12227]) forSome { type _1#12227; type e#12038 }

I suspect that this existential dropping only makes sense outside of
typechecking patterns. In pattern mode, type information flows from the
expected type onwards to the body of the case; we must not lose precision
in the types.

For SIP-18 friendly existentials, one `dropExistential` is invertable with
`makeFullyDefined`, so this hasn't been such a big problem.

The error message improvement conferred by SI-4515 took a hit.
That might be a good example to consider when reviewing this change:
Does it tell us anything interesting about this `dropExistential`
business?

1 files changed, 11 insertions, 0 deletions

diff --git a/test/files/pos/t2613.scala b/test/files/pos/t2613.scala
new file mode 100644
index 0000000000..3a64dbc282
--- /dev/null
+++ b/test/files/pos/t2613.scala
@@ -0,0 +1,11 @@
+import language.existentials
+
+object Test {
+  class Row
+
+  abstract class MyRelation [R <: Row, +Relation <: MyRelation[R, Relation]]
+
+  type M = MyRelation[R, Relation] forSome {type R <: Row; type Relation <: MyRelation[R, Relation]}
+
+  var (x,y): (String, M) = null
+}