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

[suyog sarda]

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.
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>> *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*
>> )
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>> *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
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