SI-3995 Exclude companions with an existential prefix.

In `(qual: Q).apply(expr)` where `expr` must be implictily
converted to a path dependent type `T` defined in `qual`,
we were looking for companion implicits via a path prefixed
by an existential skolem `_1`. These aren't much good to us,
as when we try to feed them into `mkAttributedQualifer`, a
crash rightly ensues.

This commit excludes companions prefixed by an existentially
bound path.

2 files changed, 31 insertions, 0 deletions

diff --git a/test/files/neg/t3995.check b/test/files/neg/t3995.check
new file mode 100644
index 0000000000..844150a528
--- /dev/null
+++ b/test/files/neg/t3995.check
@@ -0,0 +1,6 @@
+t3995.scala:24: error: type mismatch;
+ found   : String("")
+ required: _1.F0 where val _1: Lift
+  (new Lift).apply("")
+                   ^
+one error found
diff --git a/test/files/neg/t3995.scala b/test/files/neg/t3995.scala
new file mode 100644
index 0000000000..8eb4698aaa
--- /dev/null
+++ b/test/files/neg/t3995.scala
@@ -0,0 +1,25 @@
+class Lift {
+  def apply(f: F0) {}
+
+  class F0
+  object F0 {
+    implicit def f2f0(fn: String): F0 = ???
+  }
+}
+
+object Test {
+  val l = new Lift
+  val f = ""
+
+  "": l.F0 // okay
+
+  // fails trying to mkAttributedQualifier for pre = Skolem(_1 <: Lift with Singletom).F0
+  // should this even have shown up in `companionImplicitMap`? It says that:
+  //
+  // "@return For those parts that refer to classes with companion objects that
+  // can be accessed with unambiguous stable prefixes, the implicits infos
+  // which are members of these companion objects."
+  //
+  // The skolem is stable, but it doen't seem much good to us
+  (new Lift).apply("")
+}