Ticket #7279 (closed feature request: fixed)
warning for unused type variables in instance contexts; -fwarn-unreachable-type-variables?
I just spend 90 minutes tracking down what ended up being a typo introduced via find-and-replace.
I accidentally introduced a spurious constraint on an instance context:
instance (Monoid m, Context t) => Class t where …
I would like to be warned about constraints on variables that certainly have no connection to variables in the instance head. -fwarn-unreachable-type-variables?
This might also help catch those signatures where all occurrences of type variable are as index arguments to a type family, rendering the functional unusable because of "ambiguous type variable" errors at every call site.