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Subject: Re: Overloading a virtual function with a non-virtual function with a requires-clause
From: Tom Honermann (tom_at_[hidden])
Date: 2021-03-03 12:26:00


I think this falls under
https://github.com/itanium-cxx-abi/cxx-abi/issues/24. I suggest adding
the example to that issue.

Tom.

On 3/3/21 11:50 AM, v.S. F. via Std-Discussion wrote:
>
> A virtual function cannot have a trailing requires-clause. However,
> while a virtual function is present in a class or class template,
> currently there is nothing prohibiting a non-virtual non-member
> function with the same parameter-type-list, the same cv-qualifiers (if
> any) and ref-qualifier (if any), and a trialing requires-clause to be
> declared in the same class or class template, because the two
> declarations do not correspond ([basic.scope.scope]/(3.3.1)
> <https://eel.is/c++draft/basic.scope.scope#3.3.1>).
>
> So following code is currently well-formed, where both the virtual and
> the non-virtual overloads are odr-used:
>
> template<int N>
>
> struct Foo {
>
>     virtual int bar() const { return N; }
>
>     int bar() const requires (N > 0) { return 0; }
>
> };
>
> int main()
>
> {
>
>     Foo<42>{}.bar();
>
> }
>
> But such overloading cannot be properly supported on Itanium C++ ABI,
> where two Foo<42>::bar overloads share the same mangled name
> "_ZNK3FooILi42EE3barEv”. Clang 11 accepts it with option “-std=c++2a
> -O2”(https://gcc.godbolt.org/z/fP4YTf) but rejects it with option
> “-std=c++2a”(https://gcc.godbolt.org/z/rE8sjP).
>
> On the other hand, such overloading can be supported by MSVC, because
> mangled names of virtual and non-virtual functions are always
> different on MS ABI.
>
> I think this kind of overloading should be conditionally supported,
> and diagnostic should be required on platforms that cannot support it
> properly.
>
> (I have mailed to wmm_at_[hidden] <mailto:wmm_at_[hidden]>twice for this issue
> but have not been replied yet.)
>
> Jiang An
>
>



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