Confusing errors when compiling text-1.2.0.0

I was trying to install text-1.2.0.0 on my system (Mac OS X 10.7.5 Lion) through cabal (cabal-install-1.20.0.3, cabal-1.20.0.2) and got some compile errors: [29 of 43] Compiling Data.Text.IO ( Data/Text/IO.hs, dist/build/Data/Text/IO.o )

Data/Text/IO.hs:236:29:

The last statement in a 'do' block must be an expression

n1 <- writeCharBuf raw n '\r' writeCharBuf raw n1 '\n' >>= inner s'

Data/Text/IO.hs:237:49:

Not in scope: ‘n1’

Perhaps you meant one of these: ‘n’ (line 230), ‘s1’ (line 228)

Let's see what we have at 236 line of Data/Text/IO.hs:

writeBlocksCRLF :: Handle -> Buffer CharBufElem -> Stream Char -> IO ()
writeBlocksCRLF h buf0 (Stream next0 s0 _len) = outer s0 buf0
 where
  outer s1 Buffer{bufRaw=raw, bufSize=len} = inner s1 (0::Int)
   where
    inner !s !n =
      case next0 s of
        Done -> commit n False{-no flush-} True{-release-} >> return ()
        Skip s' -> inner s' n
        Yield x s'
          | n + 1 >= len -> commit n True{-needs flush-} False >>= outer s
          | x == '\n'    -> do n1 <- writeCharBuf raw n '\r' -- 236 line is here!!!
                               writeCharBuf raw n1 '\n' >>= inner s'
          | otherwise    -> writeCharBuf raw n x >>= inner s'
    commit = commitBuffer h raw len

Seems legit… And if I rewrite this two lines as

          | x == '\n'    -> do n1 <- writeCharBuf raw n '\r'; writeCharBuf raw n1 '\n' >>= inner s'

then ghc compiles them fine. Even more interesting - if I rewrite this lines as

          | x == '\n' -> do n1 <- writeCharBuf raw n '\r'
                            writeCharBuf raw n1 '\n' >>= inner s'

then it compiles them too! Again, with context, look carefully:

        Yield x s'
          | n + 1 >= len -> commit n True{-needs flush-} False >>= outer s
          | x == '\n'    -> do n1 <- writeCharBuf raw n '\r'
                               writeCharBuf raw n1 '\n' >>= inner s'
          | otherwise    -> writeCharBuf raw n x >>= inner s'

doesn't work.

        Yield x s'
          | n + 1 >= len -> commit n True{-needs flush-} False >>= outer s
          | x == '\n' -> do n1 <- writeCharBuf raw n '\r'
                            writeCharBuf raw n1 '\n' >>= inner s'
          | otherwise    -> writeCharBuf raw n x >>= inner s'

works! Weird…

P.S. GHC version 7.8.3, installed through homebrew

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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