On Mon, 29 Jan 2024, 18:24 Jonathan Wakely, <jwakely....@gmail.com> wrote:
>
>
> On Mon, 29 Jan 2024, 18:17 Martin Licht via Gcc, <gcc@gcc.gnu.org> wrote:
>
>> GCC seems to have a non-standard lenient C++ overload resolution that is
>> not documented (or easy to find in the manual).
>>
>> The following C++ code compiles with GCC. However, it produces a warning
>> about an ambiguous overload that apparently is not acceptable by the
>> standard. Clang and MVSC simply reject the code because of the ambiguous
>> overload.
>>
>> ```
>> // reduced example
>> #include <iostream>
>> using namespace std;
>> struct A{};
>> struct B: public A {};
>> struct C{};
>> struct D{ D() = default; D(const C&){}; };
>>
>> void p( A&, const C& ){ cout << "AC\n"; };
>> void p( B&, const D& ){ cout << "BD\n"; };
>>
>> int main() {
>> B b; C c;
>> p(b,c); // ambiguous call
>> return 0;
>> }
>> ```
>>
>> GCC detects ambiguity in which `p` to choose: either cast from derived
>> class B& to base class A& or construct D from C.
>>
>> ```
>> $ g++ test.cpp
>> test.cpp: In function ‘int main()’:
>> test.cpp:51:10: warning: ISO C++ says that these are ambiguous, even
>> though the worst conversion for the first is better than the worst
>> conversion for the second:
>> 51 | p(b,c);
>> | ~^~~~~
>> test.cpp:12:6: note: candidate 1: ‘void p(A&, const C&)’
>> 12 | void p( A&, const C& ){ cout << "AC\n"; };
>> | ^
>> test.cpp:15:6: note: candidate 2: ‘void p(B&, const D&)’
>> 15 | void p( B&, const D& ){ cout << "BD\n"; };
>> | ^
>> ```
>>
>> With `-pedantic` it does not compile anymore. I subjectively agree with
>> GCC that the first choice is better than the second. But not everyone
>> may do so. Is there a documentation of all these deviations from the
>> standard?
>>
>
>
> The standard says this is ill-formed and requires a diagnostic, and gcc
> gives you a diagnostic. So it's not strictly-speaking a deviation from the
> standard.
>
> I don't think there are docs for this or for other similar "pedwarn"
> extensions.
>
You can find them all by grepping the sources for pedwarn though.
>
