https://llvm.org/bugs/show_bug.cgi?id=29140
Bug ID: 29140
Summary: -Wvarargs gives false positive with enum arguments
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Frontend
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
The following code:
----------------------------
#include <stdarg.h>
enum E { A };
void foo(enum E e, ...) {
va_list arg;
va_start(arg, e);
}
----------------------------
Produces a warning since r267338:
/tmp/enum_varags.c:7:16: warning: passing an object that undergoes default
argument promotion to 'va_start' has undefined behavior [-Wvarargs]
va_start(arg, e);
^
/tmp/enum_varags.c:5:17: note: parameter of type 'enum E' is declared here
void foo(enum E e, ...) {
This is not UB because the enum is represented in a type compatible with int.
Excerpt from the mailing list conversation:
>>>> Would you agree the warning as implemented gives false positives in that
>>>> case?
>>> It depends entirely on the enumeration and what compatible type is
>>> chosen for it. If the compatible type is signed or unsigned int, then
>>> the warning would be a false-positive and we should silence it. If the
>>> compatible type is char, then the warning is not a false positive.
>> I think we agree? If I’m not mistaken, in clang’s case, an int compatible
>> type will
>> always be chosen except if -fshort-enums is passed and the enum fits in a
>> smaller
>> type. Do you want a PR?
> Yes, I think a PR is reasonable. There's also the packed attribute on
> an enum which acts the same as passing -fshort-enums, and there is the
> language extension for allowing enumerators larger than what fits into
> an int.
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