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, 5 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/UnCurry.scala b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
index 44d2469621..628090dba5 100644
--- a/src/compiler/scala/tools/nsc/transform/UnCurry.scala
+++ b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
@@ -205,8 +205,11 @@ abstract class UnCurry extends InfoTransform
      *
      */
     def transformFunction(fun: Function): Tree =
-      // Undo eta expansion for parameterless and nullary methods
-      if (fun.vparams.isEmpty && isByNameRef(fun.body)) { noApply += fun.body ; fun.body }
+      // Undo eta expansion for parameterless and nullary methods, EXCEPT if `fun` targets a SAM.
+      // Normally, we can unwrap `() => cbn` to `cbn` where `cbn` refers to a CBN argument (typically `cbn` is an Ident),
+      // because we know `cbn` will already be a `Function0` thunk. When we're targeting a SAM,
+      // the types don't align and we must preserve the function wrapper.
+      if (fun.vparams.isEmpty && isByNameRef(fun.body) && fun.attachments.get[SAMFunction].isEmpty) { noApply += fun.body ; fun.body }
       else if (forceExpandFunction || inConstructorFlag != 0) {
         // Expand the function body into an anonymous class
         gen.expandFunction(localTyper)(fun, inConstructorFlag)