[Sml-implementers] Structure sharing and type abbreviations

[David M. <Dav...@pr...>]

Dave's question reminded me that there was something that I had been
meaning to ask.  Structure sharing in ML97 is defined as a textual
expansion of type abbreviations.  So if we have
signature S = sig type s type t end
signature T = sig structure A: S and B: S sharing A=B end
then this is equivalent to
signature T = sig structure A: S and B: S sharing type A.s=B.s sharing
type A.t=B.t end

But this seems, if I have understood correctly, to have a problem if s
or t are anything but a type name.  e.g.
signature S = sig type s type t = s * s end
signature T = sig structure A: S and B: S sharing A=B end
appears to be illegal, even though its meaning is perfectly clear.  

This was actually brought to my attention some time ago by Frank
Pfenning who had some code which had compiled under SML/NJ but was
rejected by Poly/ML.  My feeling is that although this is strictly
illegal there is a strong case for allowing it on pragmatic grounds.  Is
my interpretation correct and what do other implementations do?

David.