"using the result of a comma expression is not allowed"

[Dukc]

On Sunday, 21 April 2024 at 16:28:47 UTC, Don Allen wrote:
> I just ran across "using the result of a comma expression is 
> not allowed" in a situation where I used a comma expression as 
> I would in C, expecting its value to be the value of the 
> right-most expression.
>
> While D's language document is accurate in explaining how the 
> comma expression is treated in D in 11.5, that treatment 
> conflicts with 11.1:
> "Expressions are used to compute values with a resulting type. 
> These values can then be assigned, tested, or ignored. 
> Expressions can also have side effects."
>
> This restriction makes no sense to me, nor does calling this an 
> "expression" if you can't use its value.

The comma expression used to work just like in C several years 
ago. Using it's value was disallowed at some point because there 
have been talks about using the syntax for something else - 
tuples if I recall correctly. The idea was that this way we don't 
end up changing what a syntax does overnight, it will be disabled 
for a few years in between so there can be a deprecation period. 
Apparently the spec wasn't corrected completely to reflect the 
new rules.

It is an expression in a grammatical sense, meaning you can put a 
comma expression anywhere you would put a side-effecting 
expression. For example:

```D
for(;;comma(), expression()) condition?
     another(), comma(), expression():
     (yet(), another(), expression());
```

...well it seems the compiler rejects the conditional here 
(otherwise this is fine). That's arguably a bug, after all it's 
not using the result in any way.