SI-9943 final/sealed class does not yield SAM type

Cannot subclass such a class. (Well, we could subclass a sealed
class in the same compilation unit. We ignore this for simplicity.)

This is a bit of a sneaky fix for this bug, but our hand
is pretty much forced by other constraints, in this intersection
of overload resolution involving built-in function types and SAMs,
and type inference for higher-order function literals (#5307).

Luckily, in this particular issue, the overloading clash seems
accidental. The `sealed` `<:<` class is not a SAM type as it
cannot be subclassed outside of `Predef`. For simplicity,
we don't consider where the SAM conversion occurs and exclude
all sealed classes from yielding SAM types.

Thanks to Miles for pointing out that `final` was missing in my
first iteration of this fix.

1 files changed, 1 insertions, 0 deletions

diff --git a/spec/06-expressions.md b/spec/06-expressions.md
index 2b238d149a..468d9f5fef 100644
--- a/spec/06-expressions.md
+++ b/spec/06-expressions.md
@@ -1361,6 +1361,7 @@ Note that a function literal that targets a SAM is not necessarily compiled to t
 
 It follows that:
   - if class `C` defines a constructor, it must be accessible and must define exactly one, empty, argument list;
+  - class `C` cannot be `final` or `sealed` (for simplicity we ignore the possibility of SAM conversion in the same compilation unit as the sealed class);
   - `m` cannot be polymorphic;
   - it must be possible to derive a fully-defined type `U` from `S` by inferring any unknown type parameters of `C`.