* return pointer-or-null
* choose "invalid" sentinel values and then use birthdate_is_valid(...) to check validity.
* Add an is_valid bool field (or even an error enum like in the C++23 example)
* Add an out field in the constructor function for the error code (similar to how ObjC does things).
Pointer-or-NULL doesn't work, because all pointers are nullable in C; you can always have a Foo* (NULL) that's doesn't actually point to a valid Foo.
Invalid sentinel values are definitionally values of a particular type that are invalid. Same with an is_valid field.
An out field in the constructor means that whatever you actually return in the case of an error is going to be a well-typed Foo that's invalid.
Sure, Optional is more elegant, but the end result is the same: Now none of the other code needs to validate; it's already been verified valid at all points where a parse error could have occurred.
C may not be an easy language, but with the right tooling you can make code safer, and idioms like parse-dont-validate possible.
All four of your examples are validate.
Know any languages that are worse than C at this?