First Draft: Static Single Assignment

[Jonathan M Davis]

On Friday, December 5, 2025 8:37:00 PM Mountain Standard Time Peter C via dip.development wrote:
> On Saturday, 6 December 2025 at 00:51:37 UTC, Walter Bright wrote:
> > On 12/5/2025 3:19 PM, Peter C wrote:
> >> If the compiler counts default initialization as an assignment,
> >
> > It does!
> >
> >> then I've already 'used up' my one assignment before I even
> >> touch the variable!
> >
> > Oh well!
>
> Only assignment uses the assignment operator -> '='!
>
> If there is no assignment operator involved, then it cannot
> reasonably be counted as an assignment!
>
> int x; // this is NOT assignment!

Well, if we want to get technical, talking about "static single assignment"
is kind of nonsensical considering what assignment means in most languages
- and certainly with what it means in D. The initial value that a variable
has comes from initialization or construction, not assignment. Code such as

int x = 42;

involves no assignment whatsoever. It's initialization, not assignment.
Assignment is only used when a variable is given a new value. For instance,

T t = 42;

would not use T's assignment operator if T overloaded the assignment
operator. Rather, it would use T's constructor, because the code is
equivalent to

T t = T(42);

In order to trigger assignment, = would have to be used outside a variable
declaration with code such as

t = 42;

rather than

T t = 42;

Similarly, assignment is _never_ legal with a const variable, because that
would mean mutating the variable, whereas initialization is perfectly legal,
because that's giving the variable its initial value.

So, what this final proposal is doing is actually saying that a final
variable can be initialized but it can never be assigned to and not actually
that it can be assigned to only once. The critical difference from const is
then that final is head-const rather than full, transitive const.

- Jonathan M Davis