Internal error while typechecking invalid program

While trying to reduce #12866 (closed) I encountered this program,

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleContexts #-}

module T12866 where

type Test2 a = (Eq (TestM a))

class Test a where
    type TestM :: *

which gives rise to this internal GHC error during typechecking,

$ ghc Hi2.hs

Hi2.hs:7:21: error:
    • GHC internal error: ‘TestM’ is not in scope during type checking, but it passed the renamer
      tcl_env of environment: [ap0 :-> Type variable ‘a’ = a]
    • In the first argument of ‘Eq’, namely ‘TestM a’
      In the type ‘Eq (TestM a)’
      In the type declaration for ‘Test2’

Hi2.hs:9:1: error:
    • The associated type ‘TestM’
      mentions none of the type or kind variables of the class ‘Test a’
    • In the class declaration for ‘Test’

Clearly the program is bogus, but the first error message is quite suspicious.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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