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, 83 insertions, 0 deletions

diff --git a/test/files/pos/t7285a.scala b/test/files/pos/t7285a.scala
new file mode 100644
index 0000000000..34e79c741b
--- /dev/null
+++ b/test/files/pos/t7285a.scala
@@ -0,0 +1,83 @@
+sealed abstract class Base
+
+object Test {
+  case object Up extends Base
+
+  def foo(d1: Base) =
+    d1 match {
+      case Up  =>
+    }
+
+    // Sealed subtype: ModuleTypeRef   <empty>.this.Test.Up.type
+    // Pattern:        UniqueThisType  Test.this.type
+}
+
+
+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
+        case (Up, Down)              => false
+      }
+  }
+}
+
+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 Up | Down => false
+      }
+  }
+}
+
+object Test3 {
+  sealed abstract class Base
+
+  object Base {
+    case object Down extends Base
+
+    (d1: Base, d2: Base) =>
+      (d1, d2) match {
+        case (Down, Down) => 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
+      case (Up, Down)              => false
+    }
+}