Template parameter deduction for constructors?

[Timon Gehr]

On 04/15/2013 05:18 PM, Steven Schveighoffer wrote:
> On Sun, 14 Apr 2013 11:47:47 -0400, Timon Gehr <timon.gehr at gmx.ch> wrote:
>
>> On 04/13/2013 01:00 AM, bearophile wrote:
>>> This is one of the few C++14 core language proposals that seem
>>> interesting for D:
>>>
>>> http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3602.html
>>>
>>> The purpose of this idea is to avoid the helper functions that are
>>> currently used in D to build template structs/classes. Even if this
>>> feature is restricted to only a subset of all the templated
>>> structs/classes it seems useful to avoid some boilerplate code.
>>>
>>> Is this idea adaptable to D?
>>>
>>> Bye,
>>> bearophile
>>
>> I think the differences to be accounted for are basically that D has
>> n-ary default constructors, static opCall, and static if. Furthermore,
>> with IFTI, it is possible to only specify the prefix of the template
>> arguments.
>>
>> ---
>>
>> struct S(T...){
>>      T data;
>> }
>>
>> S(1,2,3); // ?
>
> Should work.  The implicit default constructor should be treated as a
> function.
>

It cannot be treated exactly like a function because it depends on the 
struct layout. It is very well possible that the struct layout cannot be 
easily determined without instantiation. The example below demonstrates 
that it is not entirely obvious.

>>...
>>
>> ---
>>
>> struct S(T...){
>>      T data;
>>      static if(T.length){
>>          this(T x){ }
>>      }
>> }
>>
>> S(1,2,3); // ?
>
> I think this should follow IFTI rules.  If this is allowed:
>
> template foo(T...) {
>     static if(T.length)
>        void foo(T x) {}
> }

This probably shouldn't be allowed because a compiler cannot satisfy 
arbitrary expressions.

>
> Then the struct should be allowed as well.
>

It is more subtle than that. The layout of S begins with T data. Does 
this mean S(1,2,3) instantiates to S!(int,int,int)(1,2,3), as above ? 
When does this happen? Is it checked that the constructor used for 
guessing the parameters is the constructor that gets called?

Also: What happens if there is more than one constructor?

> ...