Do not generate warning in `do` when result is of type `Void`.

GHC generates warnings when do notation statements discard their result, without an explicit _ <- , unless the discarded result is of type ().

Another type which is used to indicate "no result", is Void from Data.Void. There are no meaningful values of this type, so we should also skip the warnings for such situations.

Here is an example:

import Data.Void

f :: IO Void
f = undefined

main :: IO ()
main = do f
          return ()

GHC output:

test.hs:7:11: Warning:
    A do-notation statement discarded a result of type ‘Void’
    Suppress this warning by saying ‘_ <- f’
    or by using the flag -fno-warn-unused-do-bind

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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