SI-7285 Fix match analysis with nested objects.

The fix for SI-6146 introduced `nestedMemberType` to
enumerate sealed subtypes based on the (prefixed) type
of the scrutinee and the symbols of its sealed subclasses.
That method needed to widen `ThisType(modSym)`s to
`ModuleTypeRef(modSym)` before calling `asSeenFrom`.

However, this could lead to confused in the match analysis,
which sees `ModuleTypeRef` as distinct from singleton types
on the same modules (after all, they aren't =:=). Spurious
warnings ensued.

This commit makes two changes:

 - conditionally re-narrow the result of `asSeenFrom` in `nestedMemberType`.
 - present `a.b.SomeModule.type` as `SomeModule` in warnings emitted
   by the pattern matcher.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/files/neg/t7285.check b/test/files/neg/t7285.check
new file mode 100644
index 0000000000..108f4292a8
--- /dev/null
+++ b/test/files/neg/t7285.check
@@ -0,0 +1,13 @@
+t7285.scala:15: error: match may not be exhaustive.
+It would fail on the following input: (Up, Down)
+      (d1, d2) match {
+      ^
+t7285.scala:33: error: match may not be exhaustive.
+It would fail on the following input: Down
+      (d1) match {
+       ^
+t7285.scala:51: error: match may not be exhaustive.
+It would fail on the following input: (Up, Down)
+    (d1, d2) match {
+    ^
+three errors found