C PATCH to rectify warning for character types (PR c/23087)

[Martin Sebor]

On 01/07/2016 02:19 PM, Joseph Myers wrote:
> On Thu, 7 Jan 2016, Marek Polacek wrote:
>
>> This PR points out that we issue a wrong warning message when assigning
>> two pointers when one of them is plain char.  In that case, the compiler
>> currently says that pointer targets differ in signedness.  But that is
>> not correct; char is a separate type from (un)signed char and is not
>> compatible with either.  So we should instead say that the pointer types
>> are not compatible.
>>
>> This effectively means that I'm turning a -Wpointer-sign warning into
>> a -Wincompatible-pointer-types warning.  But -Wincompatible-pointer-types
>> is a warning that is enabled by default and -Wpointer-sign is enabled by
>> -Wpedantic || -Wall.  So with this, we'd be more verbose and would warn
>> by default on e.g. "unsigned char *p = "foo";."  Not sure if that is
>> desirable at this stage, so I'm leaning towards pushing this patch for
>> GCC7.
>
> I don't think it's desirable to raise the warning for this case under
> different conditions from the warning for other signedness cases.  The
> targets do differ in signedness - it's just that the difference is between
> "plain" and "signed" or "plain" and "unsigned", not between signed and
> unsigned.

I'm sorry Joseph but I don't quite follow this argument.  Plain
char is neither a signed [integer] type nor an unsigned [integer]
type, so it can never differ in signedness from any other type.

It seems to me that by the interpretation you suggest, converting
a signed int* to unsigned long* should be controlled by -Wpointer-
sign when int and long have the same representation, and by
-Wincompatible-pointer-types otherwise.  (Which is not what GCC
does.)

In my view, Marek's change makes perfect sense because it the most
closely reflects the properties of the type.

Martin