Fixing spurious "statement is not reachable" in template code

[Timon Gehr via Digitalmars-d]

On 10/27/2015 10:33 PM, deadalnix wrote:
> On Saturday, 24 October 2015 at 17:25:23 UTC, tsbockman wrote:
>> While improving the DMD front-end's constant folding:
>>     https://github.com/D-Programming-Language/dmd/pull/5229
>> I found out about DMD issue 14835:
>>     https://issues.dlang.org/show_bug.cgi?id=14835
>>
>> Briefly:
>> ///////////////////////////////
>> module main;
>>
>> import std.stdio;
>>
>> void reachIf(bool x)()
>> {
>>     if(!x)
>>         return;
>>     writeln("reached"); // Warning: statement is not reachable
>> }
>>
>> void main(string[] args) {
>>     reachIf!true();  // prints "reached"
>>     reachIf!false(); // triggers warning
>> }
>> ///////////////////////////////
>>
>> This is, I think, a big problem.
>
> It is. This is the optimizer leaking into semantic. As far as language
> definition is concerned, this is a runtime if and changing it to a
> compile time one is an optimization.
>
> If this was a static if, then generating a error is reasonable, but with
> a regular if, it isn't.
>

if statements with constant conditions perhaps shouldn't be eliminated 
in the frontend, at least for reachability analysis within templated 
functions.