GCC incorrectly allows the following program:

consteval int foo(){
    struct A{char x;};
    struct B{char y;};
    struct C:A,B{};
    char C::*p=&A::x;
    char B::*q=static_cast<char B::*>(p);
    C c{};
    return((B&)c).*q;
}
int main(){
    return foo();
}

This program should be rejected since the static_cast has undefined behavior. The member x is neither in B, nor a class derived from B.