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.

3 files changed, 69 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
diff --git a/test/files/neg/t7285.flags b/test/files/neg/t7285.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/neg/t7285.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/neg/t7285.scala b/test/files/neg/t7285.scala
new file mode 100644
index 0000000000..14121d92b1
--- /dev/null
+++ b/test/files/neg/t7285.scala
@@ -0,0 +1,55 @@
+sealed abstract class Base
+
+
+object Test1 {
+  sealed abstract class Base
+
+  object Base {
+    case object Down extends Base {
+    }
+
+    case object Up extends Base {
+    }
+
+    (d1: Base, d2: Base) =>
+      (d1, d2) match {
+        case (Up, Up) | (Down, Down) => false
+        case (Down, Up)              => true
+      }
+  }
+}
+
+object Test2 {
+  sealed abstract class Base
+
+  object Base {
+    case object Down extends Base {
+    }
+
+    case object Up extends Base {
+    }
+
+    (d1: Base, d2: Base) =>
+      (d1) match {
+        case Test2.Base.Up => false
+      }
+  }
+}
+
+
+object Test4 {
+  sealed abstract class Base
+
+  object Base {
+    case object Down extends Base
+
+    case object Up extends Base
+  }
+
+  import Test4.Base._
+  (d1: Base, d2: Base) =>
+    (d1, d2) match {
+      case (Up, Up) | (Down, Down) => false
+      case (Down, Test4.Base.Up)   => true
+    }
+}