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, 22 insertions, 0 deletions

diff --git a/test/files/pos/t5022.scala b/test/files/pos/t5022.scala
new file mode 100644
index 0000000000..b9a085fb35
--- /dev/null
+++ b/test/files/pos/t5022.scala
@@ -0,0 +1,22 @@
+class ForSomeVsUnapply {
+  def test {
+    def makeWrap: Wrap = ???
+    def useRep[e](rep: (e, X[e])) = ()
+
+    val repUnapply = Wrap.unapply(makeWrap).get
+    useRep(repUnapply)  // okay
+
+    val Wrap(rep0) = makeWrap
+    useRep(rep0) // error
+
+    val rep = makeWrap match {
+      case Wrap(r) => r
+    };
+
+    useRep(rep) // error
+  }
+}
+
+class X[e]
+
+case class Wrap(rep: (e, X[e]) forSome { type e })