[cfe-commits] [PATCH] Fix for reinterpret_cast (bug 11747)

[Eli Friedman]

On Sun, Jan 22, 2012 at 12:31 AM, Aaron Ballman <aaron at aaronballman.com> wrote:
> On Sun, Jan 22, 2012 at 8:58 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
>> On Sat, Jan 21, 2012 at 11:38 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
>>> On Thu, Jan 19, 2012 at 12:00 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>>> On Mon, Jan 16, 2012 at 8:38 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
>>>>> On Sun, Jan 15, 2012 at 9:05 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>>>>> On Sun, Jan 15, 2012 at 1:45 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
>>>>>>> This is a patch for fixing 11747, allowing reinterpret_cast to "cast"
>>>>>>> where the source and destination types are the same as per
>>>>>>> [expr.reinterpret.cast]p2.  The fix itself is fairly simple -- the
>>>>>>> code was already there, it just needed to be moved up slightly since
>>>>>>> the previous check was for source and dest being pointers.
>>>>>>
>>>>>> C++11 [expr.reinterpret.cast]p2 is as follows:
>>>>>>
>>>>>> The reinterpret_cast operator shall not cast away constness (5.2.11).
>>>>>> An expression of integral, enumeration, pointer, or pointer-to-member
>>>>>> type can be explicitly converted to its own type; such a cast yields
>>>>>> the value of its operand.
>>>>>
>>>>> Attached is a new patch to hopefully better address this, as well as
>>>>> some additional test cases.
>>>>
>>>> +    // C++ 5.2.10p2 has a note that mentions that, subject to all other
>>>> +    // restrictions, a cast to the same type is allowed so long as it does not
>>>> +    // cast away constness. The intent is not entirely clear here, since all
>>>> +    // other paragraphs explicitly forbid casts to the same type.
>>>> +    //
>>>> +    // The only allowed types are: integral, enumeration, pointer, or
>>>> +    // pointer-to-member types.
>>>>
>>>> This comment is really unclear... you should really explicitly state
>>>> that the C++98 version is unclear, and C++11 clarifies this case.
>>>
>>> I've clarified now, thanks for pointing out the confusion!
>>>
>>>> +    } else if (SrcType->isPointerType() ||
>>>> +      SrcType->isMemberPointerType()) {
>>>>
>>>> The original code allowed "DestType->isAnyPointerType() ||
>>>> DestType->isBlockPointerType()".  I don't see any reason to restrict
>>>> Objective-C or block pointers.
>>>
>>> Nor do I, I've added this in as well.
>>>
>>>> +      if (CastsAwayConstness(Self, SrcType, DestType, /*CheckCVR=*/!CStyle,
>>>> +                           /*CheckObjCLifetime=*/CStyle)) {
>>>>
>>>> How could a cast where the source and destination types are identical
>>>> cast away constness?
>>>
>>> From my testing, the equality test does not take cv qualifiers into
>>> account.  Is there another way I should be testing for equality that
>>> does?
>>
>> "==" should definitely be taking qualifiers into account.  Do you have
>> a testcase?
>
> It was in the patch -- but you can reproduce it yourself pretty easily:
>
> void foo() {
>  const int i = 0;
>  (void)reinterpret_cast< int >( i );
> }
>
> Put a breakpoint in SemaCast.cpp on or around line 1629 after applying
> the patch and you'll see that you break into the SrcType == DestType
> block.

Isn't that just lvalue-to-rvalue conversion stripping off qualifiers?

-Eli