[Bug c++/108993] New: Value initialization does not occur for derived class with -Os, for gcc versions > 5

["daniel.gotsch at bluerivertech dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108993

            Bug ID: 108993
           Summary: Value initialization does not occur for derived class
                    with -Os, for gcc versions > 5
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: daniel.gotsch at bluerivertech dot com
  Target Milestone: ---

Minimal example:

#include <iostream>

struct Base {
    Base() {} // not a default constructor
    double x;
};

struct Derived : public Base {
    Derived() = default; // default constructor should lead to
zero-initialization
};

int main () {
    Base a{}; 
    std::cout << a.x; // Could be any value

    Derived b{};
    std::cout << b.x; // Should be set 0 but isn't set 0 when compiled with -Os
flag
}


see compiler output https://godbolt.org/z/Ecr1K9cMM

The class Derived has a default constructor which satisfies (2) of
value-initalization meaning the object should be zero-initialized.
see https://en.cppreference.com/w/cpp/language/value_initialization

Zero-initialization should zero-out the base class too, see
https://en.cppreference.com/w/cpp/language/zero_initialization