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

[Aaron Ballman]

On Sun, Jan 22, 2012 at 9:48 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
> 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?

I think you're right -- as soon as I use pointers, the equality test
fails as you predicted.  I've adjusted my patch accordingly, thanks
for the thorough review!

~Aaron
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