Checking types of constructors defined in Prelude

[ppolesiuk]

Some constructors are defined in Prelude, but used by some transformations (see TypeInference.PreludeTypes). Currently, the implementation doesn't check their types: it assumes that they are defined as usual. In extremely rare cases, these assumptions are not correct, which leads to "Internal type error" message, like in the following code:

data Option X = None | Some

let foo {?x} = x

let _ = foo {x=42}

I'm not sure if this is a right design. Handling such cases correctly would require some complicated code, just to report an error that should be never seen by the casual programmer. The "Internal type error" in such rare cases seems to be good enough. On the other hand, I treat internal type errors as a tool for developers: when they occur, something is really bad in the DBL implementation. Assuming that our implementation is correct, the user should be not able to see such an error.