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.
>
