Template Haskell, Haddock, -fwarn-missing-signatures don't mix

[George Russell]

Not so much a bug this as a whinge:

# ghc -c TH2.hs -fwarn-missing-signatures -fglasgow-exts
Loading package base ... linking ... done.
Loading package haskell98 ... linking ... done.
Loading package haskell-src ... linking ... done.

TH2.hs:6: Warning: Definition but no type signature for `a'

TH2.hs:8: Warning: Definition but no type signature for `a'

The problem here is that the type signature for 'a' is outside the
splice while the definition is inside.  This is a useful idiom for
me, since for Haddock I preprocess files to strip out Template
Haskell splices, but still want Haddock to see a type signature.
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module TH2 where

$(
   if True
      then
         [d| a = 2 |]
      else
         [d| a = 4 |]
 )

a :: Int