Wrong Eq Assignment instance

[lukemaurer]

In Data.Parameterized.Context.Unsafe, Assignment gets the instance

instance TestEquality f => Eq (Assignment f ctx) where
  x == y = isJust (testEquality x y)

The constraint TestEquality f is stronger than necessary. This is especially painful if a type containing an Assignment can't implement TestEquality, as it now can't use the derived Eq instance.

The fix isn't hard, but it will break code where people didn't bother declaring instances of both Eq and TestEquality (which should of course be easy to fix).

[robdockins]

What situation do you have where the f parameter doesn't have TestEquality?

[lukemaurer]

A domain for use with Crucible's fixpoint analysis. The type of domTop means the domain type can't carry a TypeRepr. This is the only way it's actually a problem, though, since you never really need to get a TypeRepr for an arbitrary domain value.

It's also worth considering whether fewer types should be schlepping around a TypeRepr as it is. They can take up a surprising amount of memory if you're not careful about sharing.