surprising error message with bang pattern

the following code

f3 :: [Int] -> IO Int  
f3 x = return (sum x) 

f4 :: [Int] -> IO Int  
f4 !x = return (sum x)

gives The type signature for ‘f4’ lacks an accompanying binding for the second function (the two functions are exactly the same except for the added bang). I do not understand bang patterns well, but would expect a more instructive error message. without the type definition, the second function f4 compiles with the bang).

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Linux
Architecture	x86_64 (amd64)

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surprising error message with bang pattern