# runtimeRepPrimRep panic with a representation-polymorphic newtype

The following program runs into a panic on HEAD:

```
{-# LANGUAGE DataKinds, DatatypeContexts, MagicHash, UnliftedNewtypes, TypeFamilies #-}
module EtaExpandNewtypeTF where
import Data.Kind
import GHC.Exts
type R :: Type -> RuntimeRep
type family R a where
R Float = FloatRep
R Double = DoubleRep
type F :: forall (a :: Type) -> TYPE (R a)
type family F a where
F Float = Float#
F Double = Double#
type C :: Type -> Constraint
class C a where {}
type N :: forall (a :: Type) -> TYPE (R a)
newtype C a => N a = MkN (F a)
foo1 :: C Float => F Float -> N Float
foo1 = MkN
foo2 :: C Double => () -> F Double -> N Double
foo2 _ = MkN
```

```
GHC version 9.5.20220525:
runtimeRepPrimRep
typePrimRep (F Float :: TYPE (R Float))
R Float
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler\GHC\Utils\Panic.hs:182:37 in ghc:GHC.Utils.Panic
pprPanic, called at compiler\GHC\Types\RepType.hs:614:5 in ghc:GHC.Types.RepType
runtimeRepPrimRep, called at compiler\GHC\Types\RepType.hs:582:5 in ghc:GHC.Types.RepType
kindPrimRep, called at compiler\GHC\Types\RepType.hs:534:18 in ghc:GHC.Types.RepType
typePrimRep, called at compiler\GHC\Types\RepType.hs:181:22 in ghc:GHC.Types.RepType
isZeroBitTy, called at compiler\GHC\Core\Unfold.hs:617:43 in ghc:GHC.Core.Unfold
```

We also get an error with Core Lint:

```
*** Core Lint errors : in result of Desugar (after optimization) ***
EtaExpandNewtypeTF.hs:25:1: warning:
Binder does not have a fixed runtime representation: ds_dHN :: (F Float :: TYPE
(R Float))
In the RHS of foo1 :: C Float =>
(F Float |> (TYPE (D:R:R[0]))_N)
-> (N Float |> (TYPE (D:R:R[0]))_N)
In the body of lambda with binder $dC_aH8 :: C Float
In the body of lambda with binder ds_dHN :: F Float
Substitution: [TCvSubst
In scope: InScope {}
Type env: []
Co env: []]
```

This seems to be a different issue than #21544 (closed), as changing newtype constructors to always inline fixes that bug but doesn't fix this one.