Deriving via TH (#12457)

[Sebastian Graf]

Hi Ryan and GHC devs,

I'm working on-and-off on a prototype that enables use of 
TemplateHaskell as a deriving strategy: 
https://gitlab.haskell.org/ghc/ghc/-/commit/82aea77ed908fe36bed829c9c4a01ea9b30a0181.
My current working example is:

```
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}

module T12457A where

import GHC.Internal.TH.Lib
import GHC.Internal.TH.Syntax

class C a where
   type Assoc a
   m :: a -> Int
   n :: a -> Int

instance DeriveTH C where
   deriveTH _p head = do
     let AppT (ConT t) (VarT a) = head
     x <- newName "x"
     x2 <- newName "x"
     addTopDecls =<< [d|
       $(varP x) = 12
       $(varP x2) = 23 |]
     [d|
       instance C $(varT a) => C ($(conT t) $(varT a)) where
         type Assoc ($(conT t) $(varT a)) = Char
         m :: Eq a => a -> Int
         m _ = $(varE x) + 42
         {-# INLINE m #-}
         n :: Show a => a -> Int
         n _ = $(varE x2) + 13 |]

---

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DerivingStrategies #-}

module T12457 where

import Language.Haskell.TH
import GHC.Internal.TH.Lib
import T12457A

newtype T a = T [a] deriving th C
```

I just managed to implement `GHC.Tc.Deriv.Infer.inferConstraints` for 
this mechanism (still hacky and broken in many ways) and am now stuck in 
`Deriv.genFamInst`.
I realised I would need to replicate the first half of `tcClsInstDecl` 
to implement it. Before long, I will probably also need to replicate the 
other half to check method bindings.
That leaves me wondering: Is it a good idea to integrate this new 
deriving strategy so tightly with the existing deriving framework?
I would rather just call `tcClsInstDecl`, do a bit of sanity checking 
for specified constraints in standalone deriving declarations and call 
it a day.

Given that you are the architect of our current deriving code, I hope 
you are the right person to ask for input.

Thanks,
Sebastian
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