https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102967
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Keywords| |diagnostic
Last reconfirmed| |2021-10-27
Ever confirmed|0 |1
Summary|-Waddress with nested |confusing location in
|structures: Incorrect "the |-Waddress for a
|comparison will always |subexpression of a ternary
|evaluate as 'true'" |expression
CC| |msebor at gcc dot gnu.org
--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning is intended: it points out that the second operand of the
conditional expression is necessarily true:
if ( !(pa ? &pa->c : NULL) )
^^^^^^
There's no point in testing the address of a member for equality to null
because the member of no object can reside at that address. The above can be
simplified to
if (!pa)
Below are a couple of small examples to illustrate.
I confirm this as a report of the underlining being confusing. In the message
on Godbolt:
20 | if ( !(pa ? &pa->c : NULL) ) // Macro expanded, just to check.
Still wrong.
| ^
and in the first message below it could stand to be improved to point to the
second operand rather than the first (as I did above).
I also note that the C front end diagnoses both expressions as expected but the
C++ front end only the latter one. That seems like an omission to me that
should be fixed.
$ cat a.c && gcc -S -Wall a.cstruct A { int i; };
int f (struct A *p)
{
if (p ? &p->i : 0) // -Waddress
return 0;
return 1;
}
int g (struct A *p)
{
if (&p->i) // -Waddress
return 0;
return 1;
}
a.c: In function ‘f’:
a.c:5:7: warning: the comparison will always evaluate as ‘true’ for the address
of ‘i’ will never be NULL [-Waddress]
5 | if (p ? &p->i : 0) // -Waddress
| ^
a.c:1:16: note: ‘i’ declared here
1 | struct A { int i; };
| ^
a.c: In function ‘g’:
a.c:12:7: warning: the comparison will always evaluate as ‘true’ for the
address of ‘i’ will never be NULL [-Waddress]
12 | if (&p->i) // -Waddress
| ^
a.c:1:16: note: ‘i’ declared here
1 | struct A { int i; };
| ^
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