https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91314
Bug ID: 91314
Summary: Confusing warning refers to nonexistent comma operator
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: Keith.S.Thompson at gmail dot com
Target Milestone: ---
Test case:
int main() {
int a;
&(a=0);
}
Demonstration:
$ g++ -c -Wunused-value confusing_warning.cpp
confusing_warning.cpp: In function ‘int main()’:
confusing_warning.cpp:3:7: warning: right operand of comma operator has no
effect [-Wunused-value]
3 | &(a=0);
| ^
The problem: The warning refers to a comma operator that does not
exist in the source.
Speculation (please ignore this if it's not useful):
The compiler internally generates some internal data structure that's
similar to a comma operator (something like `(a=0), &a`) and the
warning message is based on that.
I've reproduced this problem with all versions of g++ I have access
to, from 4.1.2 to 10.0.0 20190718 (experimental).
This problem was originally reported by Stefan Ram <[email protected]>
on comp.lang.c++, 2019-07-24, thread "why can't I apply a bitwise modifier
directly in a function call?".
