https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87569
Bug ID: 87569
Summary: defining type in ‘sizeof’ expression is invalid in C++
references wrong operator
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
While enhancing the C parser to recognize a new built-in similar to
alignof/sizeof/typeof I noticed the in_alignof, in_sizeof and in_typeof
handling by -Wc++-compat isn't quite right. The warning code uses the in_xxx
counters to reference the operator whose operand is a type definition, like
this:
warning_at (loc, OPT_Wc___compat,
"defining type in %qs expression is invalid in C++",
(in_sizeof
? "sizeof"
: (in_typeof ? "typeof" : "alignof")));
But this approach is inaccurate whew multiple counters are set because it fails
to consider that the current operator may be different between the same counter
values. Below is a test case that shows the problem (note the warning mentions
sizeof in both instances, even though only one of them defines a type in a
sizeof operand; the other one defines it in a typeof operand). To accurately
reflect the current operator the code would need to maintain a data structure
like a stack. It's not really a big deal but if one expects the message to
accurately reflect the operator (e.g., when writing tests for a new operator),
one realizes that this sort of accuracy is not possible with the current
design.
$ cat x.c && gcc -S -Wall -Wextra -Wc++-compat x.c
int i = sizeof (__typeof (enum { e0 })); // type defined in typeof
__typeof (sizeof (enum { e1 })) e; // type defined in sizeof
x.c:1:32: warning: defining type in ‘sizeof’ expression is invalid in C++
[-Wc++-compat]
1 | int i = sizeof (__typeof (enum { e0 })); // type defined in typeof
| ^
x.c:3:24: warning: defining type in ‘sizeof’ expression is invalid in C++
[-Wc++-compat]
3 | __typeof (sizeof (enum { e1 })) e; // type defined in sizeof
| ^
