Hi,

I've come across a few bugs in the preprocessor while converting the boost preprocessor library. This one is probably the most problematic.

In the preprocessor, if a macro argument contains whitespace then it prevents concatenation. The following example illustrates this:

    #define EMPTY
    #define CAT(x, y) CAT2(x, y)
    #define CAT2(x, y) x##y

    CAT(2 EMPTY, 2) // On DMC = 2 2
    CAT(2, EMPTY 2) // On DMC = 2 2

Both concatenations should expand to '22', but instead result in '2 2'.

Daniel