Type family equation with wrong name is silently accepted (GHC 8.6+ only)

Here's a program:

{-# LANGUAGE TypeFamilies #-}
module TypeFamilies where

data A
type family B (x :: *) :: * where
  A x = x

One would hope that GHC would reject that nonsensical equation for B that references A. On GHC 7.8 through 8.4, that is the case:

$ /opt/ghc/8.4.4/bin/ghci Bug.hs
GHCi, version 8.4.4: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling TypeFamilies     ( Bug.hs, interpreted )

Bug.hs:6:3: error:
    • Mismatched type name in type family instance.
        Expected: B
          Actual: A
    • In the type family declaration for ‘B’
  |
6 |   A x = x
  |   ^^^^^^^

But GHC 8.6.2 and HEAD actually //accept// this program! Thankfully, GHC appears to just treat A x = x as though you had written B x = x, so it's not like this breaks type safety or anything. Still, this most definitely ought to be rejected.

One interesting observation is that B having a CUSK appears to be important. If B doesn't have a CUSK, as in the following variant:

{-# LANGUAGE TypeFamilies #-}
module TypeFamilies where

data A
type family B x where
  A x = x

Then GHC properly catches the mismatched use of A:

$ /opt/ghc/8.6.2/bin/ghci Bug.hs
GHCi, version 8.6.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling TypeFamilies     ( Bug.hs, interpreted )

Bug.hs:6:3: error:
    • Mismatched type name in type family instance.
        Expected: B
          Actual: A
    • In the type family declaration for ‘B’
  |
6 |   A x = x
  |   ^^^^^^^

Trac metadata

Trac field	Value
Version	8.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture