[cfe-dev] Bug 18275 - Incorrect const qualifier behavior in definition.

[suyog sarda]

Gentle Ping ! Please help in reviewing this.


On Sun, Feb 2, 2014 at 9:52 PM, suyog sarda <sardask01 at gmail.com> wrote:

> Hi all,
>
> Thanks to Richard for pointing out the specifics.
>
> Attaching patch for PR18275. Please review and also suggest where to add
> the test case.
>
>
> --
> With regards,
> Suyog Sarda
>
>
> On Fri, Jan 31, 2014 at 12:43 AM, Richard Smith <metafoo at gmail.com> wrote:
>
>> On Thu Jan 30 2014 at 6:21:34 AM, suyog sarda <sardask01 at gmail.com>
>> wrote:
>>
>>> Ping !! Any help would be appreciated on this.
>>>
>>> On Thu, Jan 30, 2014 at 1:04 AM, suyog sarda <sardask01 at gmail.com>wrote:
>>>
>>> Hi,
>>>
>>> I was looking at bug 18275 - Incorrect const qualifier behavior in
>>> definition.
>>>
>>> The declaration of A::f has a const. The definition doesn't.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>>
>>>
>>>
>>>
>>>
>>>
>>> *template <typename T>struct A {    void f(const int);};template
>>> <typename T>void A<T>::f(int x){    x = 0;}void f(){    A<float> a;
>>> a.f(0);}*
>>>
>>> Clang produces an error:
>>>
>>> test.cpp:10:7: error: read-only variable is not assignable
>>>     x = 0;
>>>     ~ ^
>>> test.cpp:26:7: note: in instantiation of member function 'A<float>::f'
>>> requested here
>>>     a.f(0);
>>>       ^
>>>
>>> It only happens for templateclasses. In exact the same situation with a
>>> regular class everything compiles well.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> *struct B{    void f(const int);};void B::f(int x){    x = 0;} void
>>> f(){    B b;    b.f(0);}*GCC compiles both examples without an error.
>>>
>>> According to my understanding, the standard 13.1/3 states that Parameter
>>> declarations that differ only in the presence or absence of const and/or
>>> volatile are equivalent. That is, the const and volatile type-specifiers
>>> for each parameter type are ignored when determining which function
>>> is being declared, defined, or called. So clang is wrong to throw error
>>> in template case.
>>>
>>> After following the code path for above examples, i came across function
>>> name "*Sema::ActOnFunctionDeclarator*" in SemaDecl.cpp file, where
>>> everytime a function is redeclared/defined, its new declaration is created '
>>> *CreateNewFunctionDecl*', and its parameters are copied in following
>>> lines of code (l*ine number 6890 in SemaDecl.cpp in latest trunk
>>> version*)
>>>
>>>
>>>
>>>
>>>
>>>
>>> *for (unsigned i = 0, e = FTI.NumArgs; i != e; ++i) {        ParmVarDecl
>>> *Param = cast<ParmVarDecl>(FTI.ArgInfo[i].Param);
>>> assert(Param->getDeclContext() != NewFD && "Was set before ?");
>>> Param->setDeclContext(NewFD);        Params.push_back(Param);*
>>> I couldn't find exactly where the codepath gets different for above two
>>> cases. For both TC above, parameters get copied in above lines of code. Can
>>> someone help in pointing out, where does the properties of function
>>> parameters/variables are set and where does those properties get reset on
>>> redeclaration/definition? Am i going wrong somewhere in above code analysis?
>>>
>>>
>> The above is what happens when we parse a function declaration /
>> definition, but it's not the code path taken during template instantiation.
>>
>>  When we instantiate a class template, we instantiate declarations of all
>> the member functions of that class template. When we later instantiate the
>> definition of one of those functions, we do not instantiate another
>> declaration (potentially with slightly different actual parameter types);
>> instead, we reuse the prior declaration.
>>
>> Look into Sema::InstantiateFunctionDefinition, and in particular its call
>> to addInstantiatedParametersToScope, to see where we reuse the prior
>> parameters.
>>
>
>
>
> --
> With regards,
> Suyog Sarda
>



-- 
With regards,
Suyog Sarda
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