Inconsistent treatment of FlexibleInstances and MPTCs with standard vs. flexible deriving

Given {-# LANGUAGE GeneralizedNewtypeDeriving #-}, I can write

import Control.Monad.State.Strict

newtype Foo s m a = Foo (StateT s m a)
  deriving (Functor, Applicative, Monad, MonadState s)

However, if I want to use StandaloneDeriving to make the MonadState instance more explicit,

deriving instance Monad m => MonadState s (Foo s m)

I suddenly need to add FlexibleInstances and MultiParamTypeClasses.

In my personal opinion, the most sensible way to handle this is to change two things in two different directions:

1. Allow MPTC instance declarations (but not class declarations) without MultiParamTypeClasses.
2. Require FlexibleInstances for standard deriving clauses when they would be required for standalone deriving declarations.

Trac metadata

Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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