The Standard says:

To value-initialize an object of type T means:
  • - if T is a (possibly cv-qualified) class type with either no default constructor  or a default constructor that is user-provided or deleted, then the object is default-initialized;
  • - if T is a (possibly cv-qualified) class type without a user-provided or deleted default constructor, *then the object is zero-initialized* and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;
  • - ...

I'm wondering, why in C++11 an object needs to be zero-initialized (this was not the case before C++11)? I can't find a proposal/DR that changes that, but I suspect that this is to align with value-initialization of scalar types (which implies zero-initializaiton).

The problem is that this leads to suboptimal code generation: (this is a minimized example from optional implementation in abseil).

In case of empty structs, I don't think that zero-initialization (and value-initialization conceptually) is meaningful, since empty structs don't have/store any value.