checkValidType is defeated by a type synonym

GHC throws an error message on this code:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
module Bug where

f :: forall b (a :: Eq b => b). Int
f = 42

$ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive Bug.hs
GHCi, version 8.7.20181211: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:6:6: error:
    • Illegal constraint in a kind: Eq b => b
    • In the type signature: f :: forall b (a :: Eq b => b). Int
  |
6 | f :: forall b (a :: Eq b => b). Int
  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

However, it does //not// throw an error message if you hide Eq b => b behind a type synonym, as the following variant of the code above is accepted:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
module Bug where

type Foo b = Eq b => b

f :: forall b (a :: Foo b). Int
f = 42

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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