parse error on input `\' when using -cpp

[peter26]

When compiling code like:

infixl 9 \\
(\\) = undefined

causes a parser error, when compiling with the -cpp option

[simonmar]

Logged In: YES 
user_id=48280

This is a limitation in CPP, which interprets a backslash at
the end of a line as a line continuation.  You can work
around it in two ways: add a comment at the end of the line
to fool CPP, or use cpphs instead (recent cpphs's work
nicely with GHC, I believe).