Re: [PATCH 2/6] New warnings -Wstring-plus-{char, int} (PR c++/62181)

[Xi Ruoyao <ryxi at stu dot xidian dot edu dot cn>]

On 2017-06-19 10:51 -0600, Martin Sebor wrote:
> On 06/11/2017 07:32 PM, Xi Ruoyao wrote:
> > This patch adds warning option -Wstring-plus-int for C/C++.
> > 
> > gcc/ChangeLog:
> > 
> > 2017-06-12  Xi Ruoyao  <ryxi@stu.xidian.edu.cn>
> > 
> > 	* c-family/c.opt: New option -Wstring-plus-int.
> > 	* c-family/c-common.c (pointer_int_sum): Checking for
> > 	-Wstring-plus-int.
> 
> This is a very useful warning but I would suggest to word it
> in terms of what it actually does rather than what it might be
> intended to do.  E.g., for
> 
>    const char *p = "123" + 7;
> 
> issue
> 
>    warning: offset 7 exceeds the upper bound 3 of the array
> 
> rather than
> 
>    warning: adding 'int' to a string does not append to the string
> 
> (I have trouble envisioning on what grounds someone might expect
> the addition to have this effect.)

How about something like `const char *p = "123" + getchar();` ?

I'd like this for -Wstring-plus-int=1:

    warning: adding 'int' to a string does not append to the string
    [-Wstring-plus-int=]
        const char *p = "123" + 7;
                              ^
    note: offset 7 exceeds the size 4 of the string, using the result
    may lead to undefined behaviour.

(Clang permits "123" + 4 since its result is well defined in standard.
Maybe we could permit "123" + 3 only.)

For level 1 we only warn for such obvious mistakes. And for
-Wstring-plus-int=2:

    warning: adding 'int' to a string does not append to the string
    [-Wstring-plus-int=]
        const char *p = "123" + getchar();
                              ^
    note: the offset may exceed the size of the string.

(Clang also warn while it's impossible to know whether the offset
exceeds.  It seems aggressively so we can make it level 2.)

> Given that the warning only triggers when the upper bound of
> an array is exceeded I would also suggest to consider including
> the warning in -Warray-bounds.  (With that, it would be useful
> to also detect exceeding the upper bound of non-literal arrays
> as well.)

We can let -Warray-bounds enable -Wstring-plus-int=1, but not =2.
-- 
Xi Ruoyao <ryxi@stu.xidian.edu.cn>
School of Aerospace Science and Technology, Xidian University