Standalone-derived anyclass instances aren't as permissive as empty instances

Note that these are accepted:

class Foo (a :: TYPE ('TupleRep '[]))
instance Foo (a :: TYPE ('TupleRep '[]))
instance Foo (##)

But if you try to write these same instances using DeriveAnyClass and StandaloneDeriving, they'll be rejected for no good reason:

deriving instance Foo (a :: TYPE ('TupleRep '[]))

{-
    • Can't make a derived instance of ‘Foo a’:
        The last argument of the instance must be a data or newtype application
-}

deriving instance Foo (##)

{-
    • Can't make a derived instance of ‘Foo (# #)’:
        The last argument of the instance cannot be an unboxed tuple
-}

The latter error is a vestige of #12512 (closed), and should be easy enough to rectify. The former error is a bit tricker to fix, since much of the code in TcDeriv/TcDerivInfer assumes that the type to which the class is applied to in the instead head is a datatype or newtype TyCon. It's apparent that this doesn't hold true in the presence of DeriveAnyClass, however, so that code should be refactored to reflect this.

For practical reasons, however, it might be best to wait until after #12144 (closed) is fixed to tackle this one, since that splits out some logic in TcDerivInfer for DeriveAnyClass so as to make it no longer rely on the aforementioned TyCon invariant.

Trac metadata

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