Bug 15166 – [REG2.069-devel] spurious statement not reachable warning in static foreach loop

cat > bug.d << CODE template Group(T...) { alias expand = T; } private bool compare(alias Group1, alias Group2)() { foreach (index, element; Group1.expand) { static if (!is(Group1.expand[index] == Group2.expand[index])) return false; } return true; } unittest { alias a = Group!(int, double), b = Group!(double, int); static assert (!compare!(a, b)); } CODE dmd -c -w -unittest bug ---- DMD v2.069-devel-6279af3 DEBUG bug.d(13): Warning: statement is not reachable ---- This is similar to issue 14835, but now the warning is also emitted with static foreach loops.

Introduced in: https://github.com/D-Programming-Language/dmd/pull/4790 I'm not sure how we can "fix" this and issue 14835.

This is as annoying as go's stupid "unused variable" warning, at least during development. > I'm not sure how we can "fix" this and issue 14835. I guess the fix would be to mark those returns as dependent on the template types and not account for them when computing not reachable statements, but that sounds like quite a difficult change. I guess we might fix this in vibe.d but it might break a lot more code.

I think something along this line could work. override void visit(ConditionalStatement s) { if (s.condition.include(null, null)) { result = s.ifbody.blockExit(func, mustNotThrow); // mark as conditional fallthru, see Bugzilla 14835 if (!s.elsebody) result |= BEconditional; } else if (s.elsebody) result = s.elsebody.blockExit(func, mustNotThrow); else result = BEfallthru; } if (!(result & (BEfallthru | BEconditional)) && !s.comeFrom()) { if (s.blockExit(func, mustNotThrow) != BEhalt && s.hasCode()) s.warning("statement is not reachable"); } But for this to work ConditionalStatement must no longer be flattened before computing blockExit, thus making this change very big (and somewhat risky).

A workaround is to use a variable. bool res = true; foreach (index, element; Group1.expand) { static if (!is(Group1.expand[index] == Group2.expand[index])) { res = false; break; } } return res;

*** This issue has been marked as a duplicate of issue 14835 ***

(In reply to Martin Nowak from comment #4) > A workaround is to use a variable. I think this may be the right answer. It boils down to this: static if(someCondition) return false; return true; Which you would normally write with else, but it's not so simple in this case, because the "else" would be part of the loop. I'm curious why the return short-circuits the loop. In other words, why aren't all the "return false" statements besides the first one flagged for unreachability? Is it because the compiler stops generating the statements? I mean, if you rewrote as a bunch of static ifs, then wouldn't you have the same problem? Another possible answer is to do this: private bool compare(alias Group1, alias Group2)() { foreach (index, element; Group!(Group1.expand, void).expand) { static if(index == Group1.expand.length) return true; else static if (!is(Group1.expand[index] == Group2.expand[index])) return false; } }