[dcl.init.general]:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type ([class]), then
  • if T has either no default constructor ([class.default.ctor]) or a default constructor that is user-provided or deleted, then the object is default-initialized;
  • otherwise, 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;
• [...]

It seems that if T has no default constructor, then it can't meet the semantic constraints for default-initialization, and the "otherwise" branch correctly makes it ill-formed. (It doesn't matter whether zero-initialization is performed or not, since the initialization is ill-formed either way.)

CWG 1301 adds the "no default constructor" part. When CWG 1301 was adopted, the "otherwise" branch was not specified to check the semantic constraints for default-initialization (which was changed later by CWG 1507). Now that the "otherwise" branch does the right thing, I think it's not necessary to handle the "no default constructor" in the "if" branch.

Suggestion:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type ([class]), then
  • if T has either no default constructor ([class.default.ctor]) or a default constructor that is user-provided or deleted, then the object is default-initialized;
  • otherwise, 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;
• [...]