Given this code: (https://godbolt.org/z/qsaeM7)

struct Base {
    int x;
};

struct Derived : Base {
    constexpr Derived() { }
};

int main () {
    constexpr Derived kDerived;
    return kDerived.x;
}

g++ -std=c++17 -pedantic compiles the program that exits with code 0.
clang rejects it as I believe in c++17 for a constructor to be constexpr all members must be initialized before execution enters the body of the constructor.

I also believe that the program is invalid as of c++20 as all must be initialized by the end of the constexpr constructor. Still g++ -std=c++2a compiles it.

Note that if there was an uninitialized member of Derived itself, g++ would generate an error as expected.

There is a similar bug filed where g++ wrongly allows uninitialized member of anonymous struct inside a union member: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86581 . Not sure if this counts as a duplicate.