SI-7753 InstantiateDependentMap narrows type of unstable args

[Most of this comment and the initial fix were implemented by Jason Zaugg.
I just cleaned it up a bit.]

After a soundness fix in SI-3873, instantiation of dependent
method type results behaved differently depending on whether the argument
from which we were propagating information had a stable type
or not. This is particular to substitution into singleton types
over the parameter in question.

If the argument was stable, it was substituted into singleton
types, such as the one below in the prefix in `a.type#B`
(which is the longhand version of `a.B`)

  scala> class A { type B >: Null <: AnyRef }
  defined class A

  scala> object AA extends A { type B = String }
  defined object AA

  scala> def foo(a: A): a.B = null
  foo: (a: A)a.B

  scala> foo(AA)
  res0: AA.B = null

But what if it isn't stable?

  scala> foo({def a = AA; a: A { type B <: String}})
  res1: a.B = null

This commit changes that to:

  scala> foo({def a = AA; a: A { type B <: String}})
  res1: A{type B <: String}#B = null

1 files changed, 29 insertions, 0 deletions

diff --git a/test/files/pos/t8223.scala b/test/files/pos/t8223.scala
new file mode 100644
index 0000000000..52d6b0098e
--- /dev/null
+++ b/test/files/pos/t8223.scala
@@ -0,0 +1,29 @@
+package p {
+  class ViewEnv[AIn] {
+    type A = AIn
+    class SubView { def has(x: A): Boolean = ??? }
+    def get: SubView = new SubView
+  }
+
+  trait HasA { type A }
+  trait Indexable[R] extends HasA
+  class ArrayTC[AIn] extends Indexable[Array[AIn]] { type A = AIn }
+}
+
+package object p {
+  implicit def arrayTypeClass[A] : ArrayTC[A] = new ArrayTC[A]
+  object intArrayTC extends ArrayTC[Int]
+
+  type EnvAlias[W <: HasA] = ViewEnv[W#A]
+  type SubAlias[W <: HasA] = ViewEnv[W#A]#SubView
+
+  def f0[R](xs: R)(implicit tc: Indexable[R]): ViewEnv[tc.A]#SubView     = new ViewEnv[tc.A]() get
+  def f1[R](xs: R)(implicit tc: Indexable[R]): EnvAlias[tc.type]#SubView = new ViewEnv[tc.A]() get
+  def f2[R](xs: R)(implicit tc: Indexable[R]): SubAlias[tc.type]         = new ViewEnv[tc.A]() get
+
+  def g0 = f0(Array(1)) has 2                   // ok
+  def g1 = f1(Array(1)) has 2                   // ok
+  def g2 = f2(Array(1)) has 2                   // "found: Int(2), required: tc.A"
+  def g3 = f2(Array(1))(new ArrayTC[Int]) has 2 // "found: Int(2), required: tc.A"
+  def g4 = f2(Array(1))(intArrayTC) has 2       // ok
+}