SI-5639 Fix spurious discarding of implicit import

In Scala fa0cdc7b (just before 2.9.0), a regression in implicit
search, SI-2866, was discovered by Lift and fixed. The nature of
the regression was that an in-scope, non-implicit symbol failed to
shadow an eponymous implicit import.

The fix for this introduced `isQualifyingImplicit` which discards
in-scope implicits when the current `Context`'s scope contains
a name-clashing entry.

Incidentally, this proved to be a shallow solution, and we later
improved shadowing detection in SI-4270 / 9129cfe9. That picked
up cases where a locally defined symbol in an intervening scope
shadowed an implicit.

This commit includes the test case from the comments of SI-2866.
Part of it is tested as a neg test (to test reporting of ambiguities),
and the rest is tested in a run test (to test which implicits are
picked.)

However, in the test case of SI-5639, we see that the scope lookup
performend by `isQualifyingImplicit` is fooled by a "ghost" module
symbol. The apparition I'm referring to is entered when
`initializeFromClassPath` / `enterClassAndModule` encounters
a class file named 'Baz.class', and speculatively enters _both_ a
class and module symbol. AFAIK, this is done to defer parsing the
class file to determine what inside. If it happens to be a Java
compiled class, the module symbol is needed to house the static
members.

This commit adds a condition that `Symbol#exists` which shines a torch
(forces the info) in the direction of the ghost module symbol. In our
test, this causes it to vanish, as we only need a class symbol for
the Scala defined `class Baz`.

The existing `pos` test for this actually did not exercise the bug,
separate compilation is required. It was originally checked in to
`pending` with this error, and then later moved to `pos` when someone
noticed it was not failing.

4 files changed, 28 insertions, 14 deletions

diff --git a/test/files/pos/t5639/A_1.scala b/test/files/pos/t5639/A_1.scala
new file mode 100644
index 0000000000..c5da10eae4
--- /dev/null
+++ b/test/files/pos/t5639/A_1.scala
@@ -0,0 +1,17 @@
+import Implicits._
+
+class Baz
+
+object Test {
+  implicitly[Int]
+}
+
+object Implicits  {
+  implicit val Baz: Int = 0
+  // This implicit was being ignored by `isQualifyingImplicit`
+  // if the classpath contained a class file for `class Baz`.
+  // This is because the package scope contains a speculative
+  // symbol for `object Baz` which is entered by `SymbolLoaders`
+  // before looking inside the class file. (A Java originated
+  // classfile results in the class/module symbol pair.)
+}
diff --git a/test/files/pos/t5639/A_2.scala b/test/files/pos/t5639/A_2.scala
new file mode 100644
index 0000000000..2bb36273e0
--- /dev/null
+++ b/test/files/pos/t5639/A_2.scala
@@ -0,0 +1,11 @@
+import Implicits._
+
+class Baz
+
+object Test {
+  implicitly[Int]
+}
+
+object Implicits  {
+  implicit val Baz: Int = 0
+}
diff --git a/test/files/pos/t5639/Bar.scala b/test/files/pos/t5639/Bar.scala
deleted file mode 100644
index f577500acd..0000000000
--- a/test/files/pos/t5639/Bar.scala
+++ /dev/null
@@ -1,7 +0,0 @@
-package pack.age
-
-import pack.age.Implicits._
-
-object Quux {
-  def baz : Baz   = 1
-}
diff --git a/test/files/pos/t5639/Foo.scala b/test/files/pos/t5639/Foo.scala
deleted file mode 100644
index 1a07734a8e..0000000000
--- a/test/files/pos/t5639/Foo.scala
+++ /dev/null
@@ -1,7 +0,0 @@
-package pack.age
-
-class Baz
-
-object Implicits  {
-  implicit def Baz(n: Int): Baz = new Baz
-}