Inconsistent warnings for unused binders in type and instance declarations

Consider compiling this with -Wunused-matches:

class C a where
  op :: a -> a
instance C (Maybe a) where
  op x = x

We get no warnings, even though a is patently unused.

But suppose we add an associated type

class C a where
  type T a
  op :: a -> a
instance C (Maybe a) where
  type T (Maybe a) = Int  -- Warning on this line
  op x = x

Now we get a warning for an unused binding for a on the type instance.

Edward complained about this inconsistent behaviour in this email thread.

My thoughts:

• Currently GHC does not warn about type variables bound in the instance head but unused in the where part. Fixing that might be a good idea, but would be a new feature.
• However, given that we don't warn about them, we should definitely not warn about instance type variables being unused in an associated type. But we could warn about ones specific to the associated type itself. Eg

class C2 a where
  type T2 a b
instance C2 (Maybe a) where
  type T2 (Maybe a) x = Int    -- Line XXX

Here, on line XXX, we might reasonably warn about the unused x, but not about the unused a.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
CC	ekmett
Operating system
Architecture