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?
