r200521 - [Sema] For -Wnon-literal-null-conversion warning, look through integer casts, which are used

[James Dennett]

On Sat, Feb 1, 2014 at 6:05 PM, Dmitri Gribenko <gribozavr at gmail.com> wrote:
> On Fri, Jan 31, 2014 at 7:51 AM, Argyrios Kyrtzidis <akyrtzi at gmail.com> wrote:
>> Author: akirtzidis
>> Date: Fri Jan 31 01:51:32 2014
>> New Revision: 200521
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=200521&view=rev
>> Log:
>> [Sema] For -Wnon-literal-null-conversion warning, look through integer casts, which are used
>> by some projects in their null macro.
>
> Hi Argyrios,
>
> I wonder, why isn't this handled automatically by
> Expr::isNullPointerConstant -- it should be evaluating the whole
> constant expression in this case.
>
> Dmitri

I think we might not want to do this (i.e., we may wish to revert the patch).

A goal of -Wnon-literal-null-conversion is to match rules proposed for
C++14 (and possibly C++11 as a fix for core defect 903), and in those
rules only integer literals (and nullptr) are valid null pointer
constants.  The C++14 draft N3690 says "A null pointer constant is an
integer literal (2.14.2) with value zero or a prvalue of type
std::nullptr_t."

The previous implementation of -Wnon-literal-null-conversion would
warn those projects so that they could fix their code.  With this
change (r200521), their (newly) non-conforming code won't be
diagnosed.

Likely we need to improve the documentation of this diagnostic in any
case.  The best extant description seems to be the commit message from
http://llvm.org/viewvc/llvm-project?view=revision&revision=161501,
which says "...This is effectively a warning for code that violates
core issue 903 & thus will become standard error in the future,
hopefully."

-- James