[PATCH] Warn when NULL is returned from 'operator new' without 'throw()'

[Richard Smith]

Thanks!

I added some more test cases for dependent exception specifications, found
they failed, and fixed up the implementation to handle them appropriately.
I delayed the check until after we've converted the return expression to
the return type, to avoid issuing the warning if the expression is
ill-formed (as happened for the 'return 1 - 1;' testcase in C++11). I also
merged together the C++98 and C++11 test files.

With those changes, committed as r199452.


On Wed, Jan 15, 2014 at 3:00 AM, Artyom Skrobov <Artyom.Skrobov at arm.com>wrote:

> Hello Richard,
>
> Would you please be able to review my updated patch?
> (attaching it again, just in case)
>
>
>
> From: metafoo at gmail.com [mailto:metafoo at gmail.com] On Behalf Of Richard
> Smith
> Sent: 10 January 2014 23:25
> To: Artyom Skrobov
> Cc: cfe commits
> Subject: Re: [PATCH] Warn when NULL is returned from 'operator new' without
> 'throw()'
>
> On Fri, Jan 10, 2014 at 5:07 AM, Artyom Skrobov <Artyom.Skrobov at arm.com>
> wrote:
> Thank you for your suggestions Richard!
> One point though:
> > Please use RetValExp->isNullPointerConstant instead.
> > Please also add testcases for operator new returning nullptr, and
> returning expressions such as 1 - 1, and for operator new marked as
> 'noexcept'.
> I want to note that expressions such as 1-1 are invalid as return values
> from operator new, and produce "error: cannot initialize return object of
> type 'void *' with an rvalue of type 'int'"
>
> Expressions such as 1 - 1 are valid null pointer constants in C++98 but not
> in C++11.
>
> At the same time, expressions such as (void*)(1-1) are not recognized
> either
> by isNullPointerConstant or by EvaluateAsInt as integer zeroes, namely
> because they are not integers.
>
> In C++11, (void*)(1 - 1) is a reinterpret_cast of 0 to void*, and isn't
> (necessarily) a null pointer. In C++98, it's a static_cast, and is a null
> pointer.
>
> For a reference, GCC doesn't warn on
>
>    void *operator new(size_t n) {
>      return (void*)(1-1);
>    }
>
> which isn't too bad; but neither does it warn on
>
>    void *operator new(size_t n) {
>      void* blah = 0;
>      return blah;
>    }
>
> OK, I would not expect a warning here.
>
> nor even on
>
>    void *operator new(size_t n) {
>      return (void*)0;
>    }
>
> I *would* expect a warning here.
>
> which seems quite valuable to be able to detect.
>
> isNullPointerConstant doesn't recognize the two latter cases as null
> pointer
> constants, either.
>
> Sorry for sending you in the wrong direction!
>
> Do you think Clang should be able to detect such null-pointer-expressions?
> If so, could you advise how it could be implemented, seeing that
> isNullPointerConstant and EvaluateAsInt prove ineffective?
>
> It looks like the best way to achieve this is to use
> Expr::EvaluateAsBooleanCondition on the (converted) return expression. That
> matches what we do for __attribute__((nonnull)).
>
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