On Tuesday, 27 October 2015 at 17:23:21 UTC, Jonathan M Davis
wrote:
On Tuesday, 27 October 2015 at 16:05:31 UTC, tsbockman wrote:
On Tuesday, 27 October 2015 at 13:23:51 UTC, Timon Gehr wrote:
Versions of the same statement in different instantiations
are independent. Templates are just a restricted form of
hygienic macros.
That's how the current implementation works, but that doesn't
mean the warning is actually *helpful*.
Well, arguably that's exactly what templates _are_ regardless
of the implementation - i.e. a template is just a template to
generate code, not really actual code in and of itself.
That is one (valid) way of thinking about templates.
Another perspective, though, which I picked up from someone
(Andrei Alexandrescu, I think?) in the D community, is to
consider template parameters simply as additional function
arguments, which happen to be evaluated at compile time. In many
cases, the timing of their evaluation is just an implementation
detail - a performance optimization (or de-optimization, as the
case may be).
Thinking about templates in this way leads naturally to Steven
Schveighoffer's comparison with function inlining, above.
Now, that being said, I'm not sure that warning about
unreachable code in a templated function is particularly
helpful - particularly if it forces you to jump through hoops
to make the compiler shut up about it. Heck, if anything, I
find that warning/error annoying in regular code, because it
tends to get in the way of debugging while developing. It
wouldn't really hurt my feelings any if we just removed it from
the compiler entirely - though I completely agree that you
don't really want to have unreachable code left in your
production code.
- Jonathan M Davis
I find the warning helpful; it just needs to be limited to cases
in which the code is actually unreachable. Removing it entirely
wouldn't be a disaster, but we'd have to think carefully about
whether the payoff of improvements to VRP and constant folding
was really worth it.
Do you have an opinion regarding my solution #1 - defer "not
reachable" warnings until all instantiations of a template have
been analyzed, and only issue the warning if it applies to every
one of them?
