Deriving a class via an instance that has a TypeError constraint using standalone deriving fails during compilation.

This bug occurs if I define an instance for a typeclass adding a TypeError constraint on the instance, and then I try to derive that instance using Deriving Via in combination with StandaloneDeriving. This is the shortest example I was able to come up with:

{-# Language DataKinds, UndecidableInstances, StandaloneDeriving, DerivingVia #-}  

import GHC.TypeLits

newtype NotNum a = NotNum a

instance (TypeError (Text "Not a num")) => Num (NotNum a) where

data Foo = Foo
deriving via (NotNum Foo) instance Num Foo

In this case, it'll fail in compilation with 'Not a Num'.

This only seems to happen when combining deriving via with standalone deriving, because if I derive the class like:

data Foo = Foo deriving Num via (NotNum Foo)

It works as expected (doesn't fail with my custom error until I actually try to use a Foo where I should use a Num)

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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