Should this be flagged as a warning?

[Don Clugston]

On 25/09/12 21:30, Bernard Helyer wrote:
> I tried to post this last night, but the NG wasn't having any of it.
>
> I found myself writing a bug that looked like this
>
>      match(ts, TokenType.Is);
>      match(ts, TokenType.OpenParen);
>      isExp.type == parseType(ts);
>
> The bug being of course, that a type is parsed and ts is modified,
> so the expression has side effects so it's not flagged as a useless
> expression. But the comparison still has no effect, so should this be
> flagged by DMD?
>
> -Bernard.


The "must have an effect" rule only applies to statements, not 
expressions, so this is according to the spec. It's not a bug.

This is a bit like the more extreme case I recently posted about:

int x, y, z;
x == y, ++z;

doesn't generate an error message even though x == y has no 
side-effects, because comma is an expression, not a statement.

IMHO, every expression should be required to have an effect. For example
foo() + foo();
shouldn't compile, unless + is an overloaded operator with side-effects.

It would be really interesting to add this check, and then compile 
existing code (such as Phobos) to see if it breaks valid code, or 
reveals bugs.