Representation polymorphism validity check is too strict

As of 465c6c5d GHC rejects this program,

{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE RankNTypes #-}
module Hi where
import GHC.Exts
data TypeRep (a :: k) where
    TrFun :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep) (a :: TYPE r1) (b :: TYPE r2).
             TypeRep a
          -> TypeRep b
          -> TypeRep (a -> b)

Claiming that it has inappropriate polymorphism over RuntimeRep,

$ ghc hi2.hs
[1 of 1] Compiling Hi               ( hi2.hs, hi2.o )

hi2.hs:7:5: error:
    • A representation-polymorphic type is not allowed here:
        Type: a
        Kind: TYPE r1
    • In the definition of data constructor ‘TrFun’
      In the data type declaration for ‘TypeRep’

I suspect this check is unnecessarily strict since we never need to store a value of type a to represent TypeRep a at runtime.

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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