https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88727
Eric Gallager <egallager at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2019-01-09
CC| |egallager at gcc dot gnu.org
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=26581
Blocks| |87403
Summary|Diagnostics improvement: |Diagnostics improvement:
|Detection of undefined |Detection of undefined
|behaviour. Incomplete type |behaviour. Incomplete type
|in tenative definition with |in tenative definition with
|internal linkage. |internal linkage.
| |[-Wtentative-definition-inc
| |omplete-type]
Ever confirmed|0 |1
--- Comment #2 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Anders Granlund from comment #0)
> Test case (prog.c):
>
> static struct S s;
>
> int main()
> {
> }
>
> struct S { int x; };
>
> Compilation command line:
>
> gcc prog.c -Wall -Wextra -std=c11 -pedantic-errors
>
> Observed behaviour:
>
> No error messages outputes.
No errors, sure, but I do get a warning from -Wunused-variable:
$ gcc -c -Wall -Wextra -std=c11 -pedantic-errors 88727.c
88727.c:1:17: warning: 's' defined but not used [-Wunused-variable]
static struct S s;
^
$
>
> Possible improvement of behaviour:
>
> Outputing an error message about using an incomplete type in the tenative
> definition static struct S s; .
>
> The program has undefined behaviour becuase of a violation of 6.9.2/2:
>
> "If the declaration of an identifier for an object is a tentative
> definition
> and has internal linkage, the declared type shall not be an incomplete
> type."
>
> GCC detects such undefined behaviour in other cases (for example using the
> incomplete type int []). It would be good if it could also hande the case
> in
> the test case for this bug report.
>
> Note:
>
> Clang detects the undefined behaviour for this program and outputs an error
> message.
Confirmed that clang errors, its output is:
$ clang -c -Wall -Wextra -std=c11 -pedantic-errors 88727.c
88727.c:1:17: error: tentative definition of variable with internal linkage has
incomplete non-array type 'struct S'
[-Werror,-Wtentative-definition-incomplete-type]
static struct S s;
^
88727.c:1:15: note: forward declaration of 'struct S'
static struct S s;
^
1 error generated.
$
Since it's a warning-turned-into-an-error I'm making this block the new-warning
bug.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87403
[Bug 87403] [Meta-bug] Issues that suggest a new warning
