I was playing around with some Eponymous Templates. I had been under the impression that Implicit Function-Template Instantiation (IFTI) meant that you don't have to explicitly instantiate all functions. However, it seems like there are cases with eponymous templates with functions in them that you do. For instance, in the bar function below, I have to explicitly instantiate the inner function template.


template foo(T, U)
{
	T foo(T x, U y)
	{
		return x + cast(T)y;
	}
}

template bar(T)
{
	T bar(U)(T x, U y)
	{
		return x + cast(T)y;
	}
}

void main()
{
	int a = 1;
	long b = 2;
	auto x = foo(a, a);
	//auto y = bar(a, b); //error
	auto y = bar!(long)(a, b);
}

On Wednesday, 29 June 2016 at 18:59:19 UTC, jmh530 wrote:
> I was playing around with some Eponymous Templates. I had been under the impression that Implicit Function-Template Instantiation (IFTI) meant that you don't have to explicitly instantiate all functions. However, it seems like there are cases with eponymous templates with functions in them that you do. For instance, in the bar function below, I have to explicitly instantiate the inner function template.
>
>
> template foo(T, U)
> {
> 	T foo(T x, U y)
> 	{
> 		return x + cast(T)y;
> 	}
> }
>
> template bar(T)
> {
> 	T bar(U)(T x, U y)
> 	{
> 		return x + cast(T)y;
> 	}
> }
>
> void main()
> {
> 	int a = 1;
> 	long b = 2;
> 	auto x = foo(a, a);
> 	//auto y = bar(a, b); //error
> 	auto y = bar!(long)(a, b);
> }

This seems like a bug?! U can clearly be deduced. The real question, is it needed?


template bar(T, U)
{
	T bar(T x, U y)
	{
		return x + cast(T)y;
	}
}

On 06/29/2016 08:59 PM, jmh530 wrote:
> For instance, in the bar function below, I have to
> explicitly instantiate the inner function template.
>
>
[...]
>
> template bar(T)
> {
>      T bar(U)(T x, U y)
>      {
>          return x + cast(T)y;
>      }
> }
>
> void main()
> {
>      int a = 1;
>      long b = 2;
>      auto x = foo(a, a);
>      //auto y = bar(a, b); //error
>      auto y = bar!(long)(a, b);
> }

You're explicitly instantiating the outer bar there, not the inner one. The inner one is instantiated via IFTI. You're explicitly setting T = long. U = long is set implicitly from the type of b.

IFTI is defined to work on function templates. The outer bar is not a function template. It's a template for a function template. So no IFTI.

Maybe IFTI could work on templates for (templates for ...) function templates. Someone would have to consider all the details, write up an improvement proposal, get it approved, and implement it.

On Wednesday, 29 June 2016 at 21:38:23 UTC, ag0aep6g wrote:
>
> You're explicitly instantiating the outer bar there, not the inner one.

Yes, you're correct. I mixed up the inner/outer. I just thought it was something weird I had noticed.