I was about to report a bug in GHC's handling of -fno-warn-missing signatures in pragmas, but wasn't able to reproduce it. Then I noticed it: the problem was that the pragma was at the very top of a .y file, where it was simply discarded, like this:

{-# OPTIONS_GHC -fno-warn-missing-signatures #-} {- -*- Haskell -*- -}

{

Happy should probably not just silently discard pragmas.