[cfe-dev] Template default arg : Friend function cannot access private member

[suyog sarda]

Thanks John for the reply. I got your point of "context to be checked where
no special access is attached". Thanks a lot.

On Wed, Sep 25, 2013 at 1:28 PM, John McCall <rjmccall at apple.com> wrote:

> On Sep 19, 2013, at 12:16 AM, suyog sarda <sardask01 at gmail.com> wrote:
> > This is regarding a test case which involves default argument
> deduction/substitution. The test case can be summarized as :
> >
> >
> > template <class T, class = typename T::I>h(T){}
> >
> > template <class T, class = typename T::I>j(T){}
> >
> > class A
> > {
> > typedef int I;
> > friend void h<A>(A);
> >
> > };
> >
> > int main()
> > {
> > A a;
> > h(a);
> > j(a);
> > }
> >
> > gcc-4.8.1 throws error for function j, since it has been not been
> declared friend nor it is private to class A and hence violates access rule
> for
> >
> > private member I(which is valid). gcc does not throw error for function
> h since it has been declared as friend to class A and hence can access
> >
> > private member I.
> >
> > Clang throws error for both the functions. Error for function j (not
> declared friend is valid and as expected), but it throws error even for
> >
> > friend function h (error : deduction of default arg failed since I is a
> private member of class A). This violates the accessibility of the friend
> >
> > function.
> >
> > The standard says - (At 14.8.2 pt 8)
> >
> >
> > "If a substitution results in an invalid type or expression, type
> deduction fails. An in valid type or expression
> >
> > is one that would be ill-formed if written using the substituted
> arguments. [ Note: Access checking is done as
> >
> > part of the substitution process. —end note ]"
> >
> >
> > Can someone please provide inputs as to how to check where the access
> rights are being checked in clang?  I am using QT creator to debug clang
>
> Clang is actually rejecting the friend function declaration, not the call
> to h.
>
> The relevant bit from the standard is [class.access]p9:
>   The names in a default template-argument have their access checked in
> the context in which they appear rather than at any points of use of the
> default template-argument.
>
> This is the resolution to DR 401:
>   http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2609.html#401
>
> Note in particular this quote, which illuminates the reasoning behind the
> somewhat confusing standard wording:
>   We decided that template parameter default arguments should have their
> access checked in the context where they appear without special access for
> the entity declared…
>
> The context in which the template parameter default argument appears is
> the global context, which has no special access rights to A.  So Clang is
> correct to reject this.
>
> GCC is probably mistakenly checking access from the context which uses the
> default argument.  I can’t imagine that they’re really doing the heroics
> necessary to check access from the context of h<A> **in order to validate
> the declaration which makes that access well-formed**.
>
> John.




-- 
With regards,
Suyog Sarda
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