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

[Paul Backus]

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.

IIRC the rationale here was to reserve the comma operator for use 
in tuple expressions (like in Python).

If you the behavior of a C-style comma expression, you can use a 
function literal:

     // `e1, e2` becomes
     () { e1; return e2; }()

If `e2` is an lvalue, you may want to prefix the function literal 
with `ref`.