Allow PartialTypeSignatures in the instance context of a standalone deriving declaration

Currently, if you try to use a wildcard anywhere in an instance context, it will fail immediately:

$ ghci
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
λ> instance _ => Show (Maybe a)

<interactive>:1:10: error:
    Wildcard ‘_’ not allowed
      in an instance declaration for ‘Show’

But there's one situation where we could lift this restriction: the context of a standalone, derived instance declaration. That is, something like this:

deriving instance _ => Show (Maybe a)

Why? Because GHC already has the machinery needed to infer what the context should be (see this part of TcDerivInfer), so if a user turned on PartialTypeSignatures, GHC could just fill in the wildcard with the inferred constraints.

The implementation won't be //that// easy, however, since we'd also have to watch out for trickery such as:

deriving instance (C a, _) => C (T a b c)

I only mentioned putting wildcards in a derived instance //context//, because I think allowing the use of wildcards elsewhere in the instance head might be too difficult to deal with. I mean, how would you fill in this, for example?

instance (_ a)

This mini-feature has a very practical application: it would allow users to wield the flexibility of StandaloneDeriving without having to manually type in the instance context every time. That is, users could type in these instances:

deriving instance _ => Data (T a b c)
deriving instance _ => Eq (T a b c)

Instead of their fully spelled-out, more laborious counterparts. This would be crucial for Template Haskell, as its ability to infer these contexts is quite limited (see #10607 (closed) for an example where this cropped up).

Idle thought: could this be generalized to work for the instance context of //any// instance declaration (and not just derived ones)? From an outside perspective, it seems like typechecking other instances would require inferring constraints in a fashion quite similar to that of derived instances. But I am not at all familiar with that part of the typechecker, so I might be totally off here.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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