A datatype that is, and is not, an equality type?

This is more of a general SML question - apologies for hijacking the 
MLton list.

I'm confused about whether a particular datatype can be used as an 
equality type.  The following code was accepted by MLton, Poly/ML and 
SML/NJ:

   signature S =
     sig
       eqtype 'a t
     end

   structure S :> S =
     struct
       datatype 'a t = A of int
     end

which suggests that 'a t is an equality type.  However, none of them accept

   signature S =
     sig
       eqtype 'a t
       val eq : 'a t * 'b t -> bool
     end

   structure S :> S =
     struct
       datatype 'a t = A of int
       fun eq (a, b) = a = b
     end

on the grounds that 'a t is _not_ an equality type!  (HaMLet behaves 
similarly but does not give such a specific error message.)  I would be 
grateful if someone could explain what is going on here.

In the end, I worked around the problem by defining

       fun eq (A a, A b) = a = b

instead.  In my actual code, the example was not so pathological: I had 
something more like

   datatype 'a t = A of 'a ref

so the type parameter was actually used.

Phil

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