Typechecker regression: Inaccessible code in a type expected by the context

The following compiles with GHC 8.0.1 and earlier, but not with GHC HEAD:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

class Foo a where
  foo :: (a ~ Int => Int) -> a -> a
  foo _ a2 = a2

instance Foo Char

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

Bug.hs:9:10: error:
    • Couldn't match type ‘Char’ with ‘Int’
      Inaccessible code in
        a type expected by the context:
          Char ~ Int => Int
    • In the expression: Bug.$dmfoo @Char
      In an equation for ‘foo’: foo = Bug.$dmfoo @Char
      In the instance declaration for ‘Foo Char’

This causes lens-4.14 to fail to build with GHC HEAD.

The reason HEAD fails to compile lens, but GHC 8.0.1 works, is because the fix for #12220 (closed) is in HEAD, but not in 8.0.1.