preprocessor/1890: macro evaluation order prevents termination

[b dot i dot mills at massey dot ac dot nz]

>Number:         1890
>Category:       preprocessor
>Synopsis:       macro evaluation order prevents termination
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Feb 06 12:06:00 PST 2001
>Closed-Date:
>Last-Modified:
>Originator:     Bruce
>Release:        gcc version 2.96 20000731 (Red Hat Linux 7.0)
>Organization:
>Environment:

>Description:
cpp appears to expand macro arguments first, and then the 
macro. The result is that some macros don't terminate which
did terminate in earlier versions of cpp that I have used.
If outer-most-complete macro expansion is used then the 
behaviour is cleaner and more general. Eg: if leftmost 
syntactically correct macro is expanded, and then the line
is re-read the problem does not occur. I was going to use
the cpp to demonstrate some properties of macro expansion
to my class, but the version I have seems to be no good 
for this.
>How-To-Repeat:
an example of a macro that terminates if outer-most 
evaluation is used, but not if arguments are expanded first.

#define zero (0,true)
#define one inc(zero)
#define two inc(one)

#define inc (a,false)
#define head(a,b) a
#define tail(a,b) b
#define true(x,y) x
#define false(x,y) y
#define apply_head(x) head x
#define apply_tail(x) tail x
#define if(x,y,z) x(y,z)

#define add(x,y) if(apply_tail(x),y,inc(add(apply_head(x),y)))

now 

add(two,two)

terminates correctly if left-most evaluation is used, but
if the arguments are expanded first, then the add macro
goes into an infinite recursive decent.

>Fix:
if the input is not re-read, and that seems to be the case
then there really is no fix to this.
>Release-Note:
>Audit-Trail:
>Unformatted: