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.
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.
|Compiler (Type checker)