Custom Type Error not getting triggered in the nested Type function call

I have partial type function which is invoked by another type function. When the inner type function fails with TypeError, outer type function is not been able to propagate that type error to its caller. As a result of it, I'm getting following error

• No instance for (KnownSymbol (NestedPartialTF (TypeError ...)))

instead of

• Unexpected type @ NestedPartialTF: Char

{-# LANGUAGE DataKinds, TypeOperators, TypeFamilies, UndecidableInstances, ScopedTypeVariables, FlexibleContexts #-}
-- | 

module CErrs where

import GHC.TypeLits
import Data.Proxy

type family PartialTF t :: Symbol where
  PartialTF Int  = "Int"
  PartialTF Bool = "Bool"
  PartialTF a    = TypeError (Text "Unexpected type @ PartialTF: " :<>: ShowType a)

type family NestedPartialTF (tsym :: Symbol) :: Symbol where
  NestedPartialTF "Int" = "int"
  NestedPartialTF "Bool" = "bool"
  NestedPartialTF a = TypeError (Text "Unexpected type @ NestedPartialTF: " :<>: ShowType a)
testPartialTF :: forall a.(KnownSymbol (PartialTF a)) => a -> String
testPartialTF t = symbolVal (Proxy :: Proxy (PartialTF a))

--t1 = testPartialTF 'a'

{- Above code rightly fails with the following error:
    • Unexpected type: Char
    • In the expression: testPartialTF 'a'
      In an equation for ‘t1’: t1 = testPartialTF 'a'
-}

-- Bug?
testNesPartialTF :: forall a.(KnownSymbol (NestedPartialTF (PartialTF a))) => a -> String
testNesPartialTF t = symbolVal (Proxy :: Proxy (NestedPartialTF (PartialTF a)))

t2 = testNesPartialTF 'a'

{- Above code fails with the following error:
    • No instance for (KnownSymbol (NestedPartialTF (TypeError ...)))
        arising from a use of ‘testNesPartialTF’
    • In the expression: testNesPartialTF 'a'
      In an equation for ‘t2’: t2 = testNesPartialTF 'a'
-}

Trac metadata

Trac field	Value
Version	8.0.1-rc3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture