Re: [c++-concepts] code review

[Jason Merrill <jason@redhat.com>]

On 06/17/2013 02:10 PM, Andrew Sutton wrote:
>> You mean you don't need <algorithm> anymore in logic.cc?  I think we want
>> the <cstdlib> include in general if we're going to support people using the
>> C++ standard library.
>
> I don't. Those decisions are above my pay grade, so I'm doing my best
> not to make them.

If you don't need the change for concepts any more, feel free to drop it.

>> Can friend temploids be constrained?
>
> I have not thought deeply about constrained friend declarations. What
> would a friend temploid look like?

I was thinking something like

template <class T> struct A {
   T t;
  requires Addable<T>()
   friend A operator+(const A& a1, const A& a2)
   { return A(a1.t + a2.t); }
};

>> I'm not clear on the issue.  Perhaps leaving processing_template_decl alone
>> and using fold_non_dependent_expr would accomplish what you want?
>
> I don't think that will work. The problem comes from the instantiation
> of traits (and some other expressions) during constraint checking.
> When processing_template_decl is non-zero, finish_trait_expr will
> create TRAIT_EXPR nodes, which aren't handled by the constexpr
> evaluation engine.

Sure, but fold_non_dependent_expr should turn the TRAIT_EXPR into a more 
useful form.

>> Can explicit specializations have constraints, to indicate which template
>> they are specializing?
>
> Good question. I don't think so. I believe that it would be a
> specialization of the most specialized function template whose
> constraints were satisfied. So:

Makes sense.

>> Passing 'true' for require_all_args means no deduction will be done; rather,
>> all arguments must either be specified or have default arguments.
>
> I see. It seems like I should be passing false here, since I want to
> ensure that the resulting argument list can be used to instantiate the
> template.

I think true is what you want, since there are no function arguments to 
do argument deduction from.  Passing true for require_all_args 
guarantees that the result can be used to instantiate the template; 
passing false can return an incomplete set of arguments that will be 
filled in later by fn_type_unification.

Jason