C++ guys hate static_if?

[Andrei Alexandrescu]

On 3/14/13 6:02 PM, H. S. Teoh wrote:
> On Thu, Mar 14, 2013 at 05:54:05PM -0400, Andrei Alexandrescu wrote:
>> On 3/14/13 2:38 PM, Jonathan M Davis wrote:
>>> On Thursday, March 14, 2013 13:26:18 Andrei Alexandrescu wrote:
>>>> Walter has measured coverage of Phobos unittests a couple of times,
>>>> it's very high. But I agree it would be nice to have it as a target.
>>>
>>> Though this is exactly a case where 100% unit test coverage doesn't
>>> mean much.  All that means is that each path has been instantiated
>>> and run. It doesn't mean that it's covered enough possible
>>> instantiations to properly test the template.
>>
>> Concepts won't help there either.
> [...]
>
> It does help. For example, if the code wrongly assumes mutability for a
> particular template type, then it may work for most test cases (frankly,
> I find const/immutable unittest coverage in Phobos very poor) but fail
> when some daring user passes const(T) instead of T to the template. For
> example, you may have accidentally written the equivalent of:
>
> 	auto func(T)(T t) {
> 		Unqual!T u = t; //<-- spot the bug
> 		...
> 	}
>
> Under a concepts system, this would be caught early because the compiler
> would detect a concept mismatch (Unqual!T != T) when analysing the
> template code.
>
> Currently, though, if there is no unittest that tries instantiating the
> template with const(T), the bug goes undetected, because in all *tested*
> instantiations, Unqual!T == T.

Template constraints start from the most permissive end of the spectrum: 
by default there's no verification, and constraints add verification.

With typeclasses it's the other way around: by default nothing is 
allowed, so code needs to add permissions explicitly.

I agree that in that regard typeclasses are better than template 
constraints. Of course there are many other aspects to be considered 
when comparing the two.


Andrei