no runtime error for -fdefer-type-errors with TypeError constraint

Part of a testsuite, I'm using -fdefer-type-errors to check if haskell expression raises type error. However the haskell expression may be perfectly valid except for a TypeError constraint on a typeclass.

The following code:

{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE DataKinds #-}

import GHC.TypeLits

class Foo t where
  foo :: t -> t
  foo = id

instance Foo Int
instance (TypeError (Text "String does not work")) => Foo String

main :: IO ()
main = do
  putStrLn (show (foo 10 :: Int))
  putStrLn (foo "hello")

Correctly generates errors when compiled:

$ ghc ./DeferTypes.hs
[1 of 1] Compiling Main             ( DeferTypes.hs, DeferTypes.o ) [flags changed]

DeferTypes.hs:17:13: error:
    • String does not work
    • In the first argument of ‘putStrLn’, namely ‘(foo "hello")’
      In a stmt of a 'do' block: putStrLn (foo "hello")
      In the expression:
        do putStrLn (show (foo 10 :: Int))
           putStrLn (foo "hello")
   |
17 |   putStrLn (foo "hello")
   | 

And also with -fdefer-type-erros, the error is transformed into a warning:

$ ghc -fdefer-type-errors ./DeferTypes.hs
[1 of 1] Compiling Main             ( DeferTypes.hs, DeferTypes.o ) [flags changed]

DeferTypes.hs:17:13: warning: [-Wdeferred-type-errors]
    • String does not work
    • In the first argument of ‘putStrLn’, namely ‘(foo "hello")’
      In a stmt of a 'do' block: putStrLn (foo "hello")
      In the expression:
        do putStrLn (show (foo 10 :: Int))
           putStrLn (foo "hello")
   |
17 |   putStrLn (foo "hello")
   |             ^^^^^^^^^^^
Linking DeferTypes ...

However, executing the program gives no runtime error:

$ ./DeferTypes 
10
hello

I was expecting something such as:

$ ./DeferTypes
10
[a defered type error exception]

With ghc 8.6.3 from nix.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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