Improve error reporting for non-rigid GADT matches
Following suggestions from users, this patch improves the error message
when a GADT match needs a rigid type:
tcfail172.hs:19:10:
GADT pattern match in non-rigid context for `Nil'
- Solution: add a type signature
+ Probable solution: add a type signature for `is_normal'
In the pattern: Nil
In the definition of `is_normal': is_normal Nil = True
Now GHC tries to tell you what to give a type signature *for*.
Thanks to Daniel Gorin and others for the suggestions.