The following code

struct Foo{};
struct Bar{};

struct Base {
    Foo func1(const Foo , const Bar = Bar{}) const {
        return {};
    };
};

struct Derived : public Base {
    using Base::func1;
    Foo func1(const Foo ) const {
        return {};
    };
};

int main() {
    Foo foo;
    Derived der;
    der.func1(foo);
}

is rejected by ICX and clang, but accepted by GCC (up to 13.1). https://godbolt.org/z/4M3rrs3r4

The consensus at https://stackoverflow.com/questions/76517300/gcc-does-not-see-function-introduced-from-base-class-as-ambiguous seems to be that this is a bug.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84832 was shown as a possible duplicate, although the problem there wasn't ambiguity.