Why is a member not getting zero-initialized in this example?

I'd also go with compiler bug.

• I think we can all agree that b gets value-initialized (8.5.4)

• Using

  value-initialize an object of type T means:
  — if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called.

  So what should happen is first zero-initialization, then default ctors may be called

• And with the definition:

  To zero-initialize an object or reference of type T means:
  — if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;

Hence the following should happen:

1. Fill sizeof(B) with zeroes
2. Call constructor of subobject A which does nothing.

I assume this is a bug in the optimization. Compare the output of -O0 to -O1: https://godbolt.org/z/20QBoR. Without optimization the behaviour is correct. Clang on the other hand is correct in both: https://godbolt.org/z/7uhlIi

This "bug" is still present with newer standard flags in GCC: https://godbolt.org/z/ivkE5K

However I assume in C++20 B is an "aggregate" so the behavior becomes standard.

For any class, if there is a single user-defined constructor it must be used, and A(){} does not initialise i.