[MLton] bug to do with as-binding masking datatype constructor name

[Matthew Fluet]

On Wed, Apr 13, 2011 at 2:49 AM, Andreas Rossberg <rossberg at mpi-sws.org> wrote:
> On Apr 13, 2011, at 01:49, Michael Norrish wrote:
>>
>> Poly 5.3 rejects
>>
>>  datatype foo = bar of string
>>
>>  fun f x = let
>>   val bar as y = 3
>>  in
>>   bar + x
>>  end
>>
>> MLton 20091212~svn accepts it.
>>
>> Who's right?
>
> Mlton, although this is arguably a design bug of the language.

I was going to conclude that Poly/ML is right.

I don't see why rule (43) applies to the "bar as y" pattern; it is not
the case that "bar \notin Dom(C)" (because "bar" is in scope) and it
is not the case that "is of C(vid) = v" (because "bar" has constructor
status).  MLton omits those two antecedents in its implementation of
"as"-pattern elaboration.  (Similarly, HaMLet omits them.)

On the other hand, "val rec bar as y = fn() => 3" would allow changing
the identifier status of "bar" (rule (26) and comment).