Role annotation for TypeRepMap

[int-index]

Before this patch, GHC inferred the phantom role and allowed the user to implement unsafeCoerce via TypeRepMap:

unsafeCoerceTRM :: a -> b
unsafeCoerceTRM a = b
  where
    Just (Const b) = TRM.lookup @() (coerce (TRM.one @() (Const a)))

Fixes #112

[int-index]

With the annotation, GHC produces this error (as intended)

    • Couldn't match representation of type ‘b’ with that of ‘a’
        arising from a use of ‘coerce’
      ‘b’ is a rigid type variable bound by
        the type signature for:
          unsafeCoerceTRM :: forall a b. a -> b
        at test/Test.hs:29:1-25
      ‘a’ is a rigid type variable bound by
        the type signature for:
          unsafeCoerceTRM :: forall a b. a -> b
        at test/Test.hs:29:1-25

[chshersh]

Looks good to me 👍🏻

[int-index]

@chshersh Perhaps you assumed I have merge permissions, but I do not.