macros: "do-while" versus "({ })" and a compile-time error

["Robert P. J. Day" <rpjday@mindspring.com>]

  just to stir the pot a bit regarding the discussion of the two
different ways to define macros, i've just noticed that the "({ })"
notation is not universally acceptable.  i've seen examples where
using that notation causes gcc to produce:

  error: braced-group within expression allowed only inside a function

i wasn't aware that there were limits on this notation.  can someone
clarify this?  under what circumstances *can't* you use that notation?
thanks.

rday