Optimizer Casting Caf with nominal type

When you make a function with a constant result, the optimizer will turn it into a cast of a CAF, even if doing so coerces a nominal parameter into a different one.

Minimal Example program:

{-# LANGUAGE RoleAnnotations #-}
import Debug.Trace (trace)
type role Nom nominal
data Nom a = Nom Int deriving Show
class Gen g where
 gen :: g
instance Gen (Nom a) where
 gen = trace "genD" $ Nom 0

main = print (gen :: Nom Int) >> print (gen :: Nom ()) >> print (gen :: Nom Char)

This program shows that only one value of type Nom is created and shared, even though doing so requires coercing a nominal role

I discovered this while checking core for sharing after creating a constraint result caching mechanism. An IOref ended up being shared for multiple different constraint result holders

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture