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.

3 files changed, 34 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index fc10f68454..68db812ad7 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -935,8 +935,8 @@ trait Implicits {
      *    - for alias types and abstract types, we take instead the parts
      *    - of their upper bounds.
      *  @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.
+     *  can be accessed with unambiguous stable prefixes that are not existentially
+     *  bound, the implicits infos which are members of these companion objects.
      */
     private def companionImplicitMap(tp: Type): InfoMap = {
 
@@ -952,7 +952,7 @@ trait Implicits {
                 infoMap(sym) = List() // ambiguous prefix - ignore implicit members
               }
             case None =>
-              if (pre.isStable) {
+              if (pre.isStable && !pre.typeSymbol.isExistentiallyBound) {
                 val companion = companionSymbolOf(sym, context)
                 companion.moduleClass match {
                   case mc: ModuleClassSymbol =>
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("")
+}