But this is the exact opposite of what you convinced me would be the right thing to do? Why generate constants specifically when you can generate arbitrary terms and that would generate constants too? We'll need to amend the AST generators to generate constants of polymorphic built-in types though, but you could just copy your constant generation code there.

So with my version we had two functions:

genConstant :: forall a. TypeRep a -> Gen a

genArg
    :: term ~ Term TyName Name DefaultUni DefaultFun ()
    => MakeKnownIn DefaultUni term a => TypeRep a -> Gen term

The former is strictly for well-typed things and the latter is for any term (deferring to the former when we need to generate a constant). Crucially, when we have SomeConstant, we need to occasionally generate arbitrary terms, which doesn't fit into the TypeRep a -> Gen a interface, but fits perfectly well into the TypeRep a -> Gen term one.

And Opaque should be handled the same way as SomeConstant.

With the version from this PR genTypeable is not always for well-typed things (i.e. it's going to be hard to control the proportion of the happy path vs unhappy path tests) and we never generate arbitrary terms for SomeConstant arguments.