For the following C++ program Clang incorrectly prints 0 instead of 1 with -O2 -fstrict-enums (see https://godbolt.org/z/Kaec7c4Eb):

#include<iostream>
#include<type_traits>

enum E { E0 } e;
enum F { F0, F1, F2, F3 };
static_assert(std::is_same_v<std::underlying_type_t<E>, std::underlying_type_t<F>>);

struct A { E e; };
struct B { F f; };
union U {
    A a;
    B b;
} u;

bool test() {
    return u.a.e == 2;
}

auto ptest = test;

int main() {
    u.b.f = F2;
    std::cout << ptest();
}

Because the two enumerations have the same underlying type, they are layout-compatible and therefore it is permitted to use the inactive u.a.e to read the active u.b.f corresponding to it in the common initial sequence. The read should happen as if u.b.f was nominated, meaning it should read the value 2 even if E is not able to represent that value.

I am not sure whether this is intentional or should be considered a defect in the standard, though.