Keep Function when CBN arg thunk targets a SAM

The body of `def delay[T](v: => T) = (v _): F0[T]`
becomes `() => v` during `typedEta`, and then uncurry
considers whether to strip the function wrapper since
`v` is known to be a `Function0` thunk. Stripping is sound
when the expected type is `Function0` for this expression,
but that's no longer a given, since we could be expecting any
nullary SAM.

Also sweep up a bit around `typedEta`.

Encapsulate the, erm, creative  encoding of
`m _` as `Typed(m, Function(Nil, EmptyTree))`.

1 files changed, 9 insertions, 0 deletions

diff --git a/test/files/run/sammy_cbn.scala b/test/files/run/sammy_cbn.scala
new file mode 100644
index 0000000000..b84b2fd8e5
--- /dev/null
+++ b/test/files/run/sammy_cbn.scala
@@ -0,0 +1,9 @@
+trait F0[T] { def apply(): T }
+
+object Test extends App {
+  def delay[T](v: => T) = (v _): F0[T]
+
+  // should not fail with ClassCastException: $$Lambda$6279/897871870 cannot be cast to F0
+  // (also, should not say boe!)
+  delay(println("boe!"))
+}