Why does gcc4 dislike this code?

[walt <wa1ter@xxxxxxxxxxxxx>]

Hi all,

This is a snippet of code that compiles nicely with gcc3 but not
with gcc4 (4.1.2 to be exact):

 *((unsigned long int *) tmp)++ = *((unsigned long int *) b1)++;

error: invalid lvalue in increment

Aside from the obvious question of why?, where could I go to find
the answer for myself?  I've looked through the Changelogs for
the early versions of gcc4 but so far I've not spotted anything
to explain the reason for this error message.

Thanks for any hints.