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.

1 files changed, 17 insertions, 0 deletions

diff --git a/test/files/neg/t2866.check b/test/files/neg/t2866.check
new file mode 100644
index 0000000000..340fb8da22
--- /dev/null
+++ b/test/files/neg/t2866.check
@@ -0,0 +1,17 @@
+t2866.scala:30: warning: imported `one' is permanently hidden by definition of value one
+    import A.one // warning: imported `one' is permanently hidden by definition of value one.
+             ^
+t2866.scala:42: error: ambiguous implicit values:
+ both value two of type Int
+ and value one in object A of type => Int
+ match expected type Int
+    assert(implicitly[Int] == 2) // !!! Not ambiguous in 2.8.0. Ambigous in 2.7.6
+                     ^
+t2866.scala:50: error: ambiguous implicit values:
+ both value two of type Int
+ and value one in object A of type => Int
+ match expected type Int
+    assert(implicitly[Int] == 2) // !!! Not ambiguous in 2.8.0. Ambiguous in 2.7.6
+                     ^
+one warning found
+two errors found