[Bug c++/102407] New: Ambiguity is not reported in case of separate inheritance of `<` and `<=>` operators

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102407

            Bug ID: 102407
           Summary: Ambiguity is not reported in case of separate
                    inheritance of `<` and `<=>` operators
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The following code is ill-formed:
```
#include <compare>

struct A { 
    bool operator <(const A&) const { return true; }
};
struct B { 
    auto operator <=>(const B&) const = default; 
};
struct C : A, B {};
int main() { 
    C c;
    c.operator<(c);
    return c < c;
}

```
and it shall be rejected due to ambiguity, but it not in GCC (neither in other
compilers). Demo: https://gcc.godbolt.org/z/d6G7e9319

The detailed explanation why is here:
https://stackoverflow.com/questions/69240507/ambiguity-in-case-of-multiple-inheritance-and-spaceship-operator-in-c20/69245639#69245639