template auto deduction

[Steven Schveighoffer]

On 10/6/17 5:08 PM, Alex wrote:
> Hey, template gurus :)
> 
> given this:
> 
> struct M(alias I : S!E, alias S, E...)
> {
>      R!E r;
>      this(S)(S initStruct)    // line 4
>      {
>          r = R!E(initStruct);
>      }
> }
> struct R(E...)
> {
>      this(S)(S initStruct)    // line 12
>      {
>          // do some cool stuff
>      }
> }
> void main()
> {
>      FS!(Etype1) fs;
>      auto m = M!(typeof(fs))(fs);    // line 21.
>      auto a = M!(fs); // does not work
>      auto b = M(fs); // does not work
> }
> struct FS(T...){}
> struct Etype1{}
> 
> Everything works as expected, especially line 21. The question is about 
> syntactic sugar: What I have to change, to use auto deduction and to 
> create the M struct like in line 22 or 23?
> 
> By the way, I'm aware, that the type matching in lines 4 and 12 is lost, 
> in the way it is written here. However, it is meant to exist, if this 
> helps in some way...
> 
> Thanks a lot in advance
> Alex

What you need is IFTI or "Implicit Function Template Instantiation"

Note the "Function" part of it, in that it's only valid for functions.

So you need a factory function:

auto m(T)(T x)
{
   return M!(T)(x);
}

...

auto b = m(fs); // ok

There is an enhancement request to make constructors have the same 
mechanism. Not sure if or when it would be implemented.

-Steve