[BUG] Superclass constraints on instances get lost with data-dependencies

[cocreature]

I admit I don’t really understand what is failing here so let me start with an example:
Dependency package

-- Copyright (c) 2018-2020, Digital Asset (Switzerland) GmbH and/or its affiliates.
-- All rights reserved.

{-# LANGUAGE UndecidableInstances #-}

module Overlap where

class C a where
  unit : a -> ()

instance C () where
  unit _ = ()

type Syn r a = (C a, DA.Internal.Record.HasField "i" r a)

class D a where
    f : a -> ()

instance Syn r a => D r where
    f _ = ()

Main package which depends on the one above via a data-dependency

module Main where

import Overlap

data T = T with
  i : ()

x : ()
x = f (T ())

This fails in the LF typechecker (the Daml typechecker is happy)

File:     daml/Main.daml
Hidden:   no
Range:    1:1-2:1
Source:   DAML-LF typechecker
Severity: DsError
Message: 
  error type checking value Main.x:
  type mismatch:
  * expected type:
  <s_1 : <m_unit : Unit -> Unit -> Unit>;
  s_2 :
  <m_getField : Unit -> Main:T -> Unit;
  m_setField : Unit -> Unit -> Main:T -> Main:T>>
  * found type:
  <m_unit : Unit -> Unit -> Unit>
  * expression:
  676e9493c4ffe6ba1ed4e4e1a58c7e8b237a44cdb6ef2c23d9ccbf4c0994627a:Overlap:$fC()

You can see that the s_1 superclass constraint got inlined while s_2 is missing.

Looking at the generated stub code we have

instance (forall r a.
          ((Overlap.C a),
           (Pkg_83623d7b8279455f5257281ab8ec1f9a2c46c61ba44e04db94d69f98c2b74fbf.DA.Internal.Record.HasField "i" r a)) =>
          Overlap.D r) where
  f _
    = (CurrentSdk.GHC.Err.error
         (CurrentSdk.Data.String.fromString "data-dependency stub"))

What seems to be happening is that the constraint synonym gets inlined and we turn the instance for the tuple into two separate instances and then we try to typecheck the first instance against the tuple for the constraint synonym.