SI-6925 use concrete type in applyOrElse's match's selector

Fix a regression introduced in 28483739c3:

PartialFunction synthesis was broken so that we'd get:
```
scala> def f[T](xs: Set[T]) = xs collect { case x => x }
f: [T](xs: Set[T])scala.collection.immutable.Set[_ <: T]
```

rather than
```
scala> def f[T](xs: Set[T]) = xs collect { case x => x }
f: [T](xs: Set[T])scala.collection.immutable.Set[T]
```

1 files changed, 9 insertions, 0 deletions

diff --git a/test/files/pos/t6925.scala b/test/files/pos/t6925.scala
new file mode 100644
index 0000000000..862a6e9d0e
--- /dev/null
+++ b/test/files/pos/t6925.scala
@@ -0,0 +1,9 @@
+class Test {
+  def f[T](xs: Set[T]) /* no expected type to trigger inference */ =
+    xs collect { case x => x }
+
+  def g[T](xs: Set[T]): Set[T] = f[T](xs) // check that f's inferred type is Set[T]
+
+  // check that this type checks:
+  List(1).flatMap(n => Set(1).collect { case w => w })
+}
\ No newline at end of file