Improve error message for unacceptable role annotations

It would be nice if this message:

Role mismatch on variable a:
  Annotation says representational but role nominal is required
while checking a role annotation for `Point'

Could be extended to say where the requirement arises.

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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changed weight to 3

added Tfeature request Trac import labels

Can you give a concrete example?

Trac metadata

Trac field	Value
Component	Compiler → Compiler (Type checker)

added typechecker label

I'm sorry guys, I haven't been haskelling much in the last couple of months.

I can't remember how to construct an example, in part because I am still confused by what the rules are, which is why I had trouble interpreting the original error message.

To elaborate on what I find confusing about it, I think that it would help if it said *where* a was used in a context that required a nominal type role, in the same way that some of the other type error messages do.

Annotation says representation but role nominal is required, arising from use of (type constructor) in the type expression (...) at (...)

I have a terrible headache or I would research the rules again to figure out how to write a program that produces this error on purpose. I'll attempt it tomorrow if there's time.

This is appropriate for a newcomer to GHC, but who has a decent understanding of roles. I am happy to advise.

added newcomer label

goldfire: I'm removing the newcomer keyword from this ticket, as it didn't see any takers in 15 months. It might be too difficult or uninteresting, but please put the keyword back if you disagree.

By keeping the newcomers list fresh, I hope we will see newcomer activity pick up a little.

added Plow label

mentioned in issue #23923 (closed)