Possible program should type check but does not using Implicit Parameters and Vectors

I think (but I'm not sure) that the following code should type check. This will require using the 'vector' package to illustrate this issue, sorry I couldn't create an example without it.

Either uncommenting the explicit m () signature or changing the type to the simpler D m data type will allow the code to compile.

{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE KindSignatures #-}

import Data.Vector.Unboxed.Mutable (MVector)
import Control.Monad.Primitive (PrimMonad, PrimState)

data D (x :: * -> *)

type T m = MVector (PrimState m) Int
--type T m = D m

h :: forall m. PrimMonad m => T m -> m ()
h x = let f _ = (let ?v = x in g) {- :: m () -} in f undefined

g :: (PrimMonad m , ?v :: T m) => m ()
g = undefined

main = pure ()

Alternatively, including g into the let binding as opposed to the top level will also compile:

{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE KindSignatures #-}

import Data.Vector.Unboxed.Mutable (MVector)
import Control.Monad.Primitive (PrimMonad, PrimState)

data D (x :: * -> *)

type T m = MVector (PrimState m) Int
--type T m = D m

h :: forall m. PrimMonad m => T m -> m ()
h x = 
  let 
    f _ = (let ?v = x in g) {- :: m () -} 
    g :: (PrimMonad m , ?v :: T m) => m ()
    g = undefined  
  in f undefined

main = pure ()

Perhaps this isn't a bug and has to do with the rank-2 type inside of Vector, but I'm just putting in this bug report to confirm.

This is an issue as of 8.0.2-rc2.

Trac metadata

Trac field	Value
Version	8.0.2-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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