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Re: Is the local class of function template be considered as dependent type

From: Christopher Hallock <christopherhallock_at_[hidden]>
Date: Fri, 21 Aug 2020 05:21:09 -0400
On Thu, Aug 20, 2020 at 11:36 PM jim x via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> Hi, everyone. please consider the below code:
> ````
> template<typename T>
> void func(){
> struct Test{};
> show(Test{});
> }
> void show(...){
> }
> int main(){
> func<int>();
> }
> ````
> There is no any rule in [temp.dep.type] to specify the local class
> declared within function template may be considered as dependent type.
> So, I think the name lookup for `show` should be bound at the point it
> is used. The second phase name lookup shouldn't occur due to `show` is
> not a dependent-name, at least according to the definition of
> dependent-name in section [temp.dep]. However, current implementations
> all consider `show` as a dependent-name which will be found by ADL in
> the context of instantiation, that is, The local class `Test` is
> viewed as dependent-type. If I don't misunderstand the section
> [temp.dep.type], how would `Test` be a dependent-type? For such a
> local class, Is it a case missed in the section [temp.dep.type].
> Thanks for discussing such an example.
> --
> Std-Discussion mailing list
> Std-Discussion_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion

I agree that the current wording does not make "Test" a dependent type in
your example. GCC/Clang/MSVC violate [temp.res]/10
<https://timsong-cpp.github.io/cppwp/temp.res#10> by failing to give an
undeclared-identifier error for "show".

But maybe that's intentional, and it's the wording that needs to be
changed. If nested classes in class templates are dependent, why not also
local classes in function templates?

Received on 2020-08-21 04:24:49