Infinite typechecker loop in GHC 8.6

The following program loops infinitely during typechecking with GHC 8.6.1 and HEAD:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Bug where

import Data.Kind

class Generic a where
  type Rep a :: Type

class PGeneric a where
  type To a (x :: Rep a) :: a

type family MDefault (x :: a) :: a where
  MDefault x = To (M x)

class C a where
  type M (x :: a) :: a
  type M (x :: a) = MDefault x

In GHC 8.4.3, however this fails with a proper error:

$ /opt/ghc/8.4.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:15:16: error:
    • Occurs check: cannot construct the infinite kind:
        a ~ Rep (M x) -> M x
    • In the type ‘To (M x)’
      In the type family declaration for ‘MDefault’
    • Type variable kinds: x :: a
   |
15 |   MDefault x = To (M x)
   |                ^^^^^^^^

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.6.1

changed weight to 7

added Tbug Trac import typechecker labels

This regression was introduced in commit faec8d35 (Track type variable scope more carefully.).

Trac metadata

Trac field	Value
Version	8.4.3 → 8.5

I really nailed this one. See the patch. :)

Trac metadata

Trac field	Value
Differential revisions	- → D4974

The patch now validates, but waiting for Simon's review before committing.

Richard and I discussed yesterday; he has a bit more to do and then will commmit.

mentioned in commit f8618a9b

Fixed now. Though this touches quite a few files, I don't think it should cause merging trouble. But it's not really critical.

Trac metadata

Trac field	Value
Test case	- → dependent/should_fail/T15380

added 1 deleted label

mentioned in issue #15464

Posted #15464 as the follow-up TH bug.

closed

Merged to ghc-8.6 with 59f38587.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

mentioned in issue #15666 (closed)

added TypeFamilies TypeInType labels

added compiler perf label

added Phigh label