At the point at which you capture g by reference, it has been declared, so the name is available for use:
3.3.2/1 The point of declaration for a name is immediately after its complete declarator (Clause 8) and before its initializer
You are allowed to use objects in limited ways before they are initialised – basically, anything that doesn’t depend on the value is OK:
3.8/6 before the lifetime of an object has started but after the storage which the object will occupy
has been allocated […] any glvalue that refers to the original object may be used but only in limited
ways. […] using the properties of the glvalue that do not depend on its value is well-defined.
So by my understanding, what you are doing is well-defined.
(Although, being ultrapedantic, I don’t think it’s specified when the storage for an automatic object is allocated, and 8.3.2/5 says that “a reference shall be initialized to refer to a valid object” without defining “valid”, so there’s scope to argue that it’s not well-defined).