Consider making throw and throwIO strict

It's possible for code to throw an exception that itself throws an imprecise exception. Such an exception is tricky to catch. For example:

import Control.Exception

strange = throwIO (undefined :: SomeException) `catch` \ex ->
  case () of
    _ | Just _ <- (fromException ex :: Maybe IOError) -> print "IOError"
      | otherwise -> print "Something else"

You might think that this would catch the exception and print "Something else", but in fact it does not. If others think this is as surprising as I do, perhaps we should make throwIO and throw strict, so an exception will never itself be bottom. Using

throwIO' !e = throwIO e

in the code above instead of throwIO allows the exception to be caught.

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Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
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