Use convert-ref-init-val to convert a U to a T& and initialize value with its
address.

GCC 15 correctly notes that 0xCAFEBABE is an unsigned int as an auto.

LEWG had several suggestions for wording.

Use conditional operator instead of if else.

- [X]  in p7 what is U? There is something missing
- [X]  it should be invoke_result_t<F>
- [X]  the definition of U should be consistent
- [X]  in p4: we shouldn’t use trailing return types here

Now that "contains a value" is defined for reference optionals.

Update code, also

Add delegation for optional(nullopt_t) constructor.

Tref gets lost.

Fix include/beman/optional/optional.hpp:1368: optioanl ==> optional

Pin one side of value_or ternary to prevent ambiguity in conversions.

Style issue.

Should be "models invocable" not "models invocable<>"

1.  missing expos italization of val in contained value defn for optional<T&>

2. In iterator begin() since contained value is now a reference, change the
iterator referring to to be to *val

3. Add mandates for transform to require that the result of invoking the
supplied function to the contained value can create an object.

P2988R12