typeclass polymorphism defeats bang patterns

I think the following program should crash, but it doesn't:

{-# LANGUAGE BangPatterns #-}
main = let n :: Num a => a; !n = undefined in return ()

Interestingly, my attempt to translate this out of BangPatterns into standard Haskell according to the translation on the wiki page led to a compile-time error:

main = let n :: Num a => a; x@n = undefined in x `seq` return ()

Produces the error:

test.hs:1:12: error:
    • Overloaded signature conflicts with monomorphism restriction
        n :: forall a. Num a => a
    • In the expression:
        let
          n :: Num a => a
          x@n = undefined
        in x `seq` return ()
      In an equation for ‘main’:
          main
            = let
                n :: Num a => a
                x@n = undefined
              in x `seq` return ()
  |
1 | main = let n :: Num a => a; x@n = undefined in x `seq` return ()
  |            ^^^^^^^^^^^^^^^

Even giving x its own explicitly polymorphic type signature -- what I thought was the standard way to prevent the monomorphism restriction from being a problem -- doesn't help. I'm uncertain what to make of that, but it certainly seems related: earlier versions of the compiler both do not give that error and do produce a crashing program, as I expected.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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