Top-level bang pattern accepted

When I compile/link

{-# LANGUAGE BangPatterns #-}

module Main where

!(Just x) = Nothing

main = putStrLn "hi there!"

I get

rae:09:50:49 ~/temp> ghc Bug.hs
[1 of 1] Compiling Main             ( Bug.hs, Bug.o )
Linking Bug ...
ld: can't open output file for writing: Bug, errno=21 for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
`gcc' failed in phase `Linker'. (Exit code: 1)

This bogus code should be rejected more gracefully (and earlier). It actually also loads into GHCi, but shouldn't.

Will fix while I'm in the area.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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