[GHC] #11990: Custom Type Error not getting triggered in the nested Type function call

[GHC]

#11990: Custom Type Error not getting triggered in the nested Type function call
-------------------------------------+-------------------------------------
           Reporter:  magesh.b       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 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

 {{{#!hs
 {-# 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'
 -}

 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11990>
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