Try to compile this code snippet:

import std.traits;
template a(R) {
	auto a(S)(auto ref R i) {
		return cast(S)i*2;
	}
}
template ReturnTypeEx(alias A, B) {
	alias ReturnTypeEx = ReturnType!(A!B);
}
template b(alias R) {
	int b(S)(S i) {
		alias Ra = ReturnTypeEx!(R, S);
		return cast(int)R!S(i);
	}
}
void main() {
	alias bb = b!(a!ulong);
	pragma(msg, ReturnTypeEx!(bb, int));
}

DMD reports:
sadf.d(3): Error: auto can only be used for template function parameters
sadf.d(8): Error: template instance sadf.a!ulong.A!int error instantiating
sadf.d(12):        instantiated from here: ReturnTypeEx!(a, int)
sadf.d(8):        instantiated from here: A!int
sadf.d(18):        instantiated from here: ReturnTypeEx!(b, int)
sadf.d(18):        while evaluating pragma(msg, ReturnTypeEx!(b, int))

But a(S)(auto ref R) is indeed a template function, so I don't understand.

On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
> Try to compile this code snippet:
>
> import std.traits;
> template a(R) {
> 	auto a(S)(auto ref R i) {
> 		return cast(S)i*2;
> 	}
> }
> template ReturnTypeEx(alias A, B) {
> 	alias ReturnTypeEx = ReturnType!(A!B);
> }
> template b(alias R) {
> 	int b(S)(S i) {
> 		alias Ra = ReturnTypeEx!(R, S);
> 		return cast(int)R!S(i);
> 	}
> }
> void main() {
> 	alias bb = b!(a!ulong);
> 	pragma(msg, ReturnTypeEx!(bb, int));
> }
>
> DMD reports:
> sadf.d(3): Error: auto can only be used for template function parameters
> sadf.d(8): Error: template instance sadf.a!ulong.A!int error instantiating
> sadf.d(12):        instantiated from here: ReturnTypeEx!(a, int)
> sadf.d(8):        instantiated from here: A!int
> sadf.d(18):        instantiated from here: ReturnTypeEx!(b, int)
> sadf.d(18):        while evaluating pragma(msg, ReturnTypeEx!(b, int))
>
> But a(S)(auto ref R) is indeed a template function, so I don't understand.

Another thing I discovered is instantiation like this: (a!T)!S will result in "C style cast illegal", which is clearly wrong because this is not a cast, (a!T) is not necessarily a type.

On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
> auto ref R) is indeed a template function, so I don't understand.

But R is not a parameter on the function itself. It comes from the outside template.

Move it to the inside template, rewrite it as:

	auto a(S, R)(auto ref R i) {
		return cast(S)i*2;
	}

and you should get further.

On Sunday, 21 June 2015 at 01:26:51 UTC, Adam D. Ruppe wrote:
> On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
>> auto ref R) is indeed a template function, so I don't understand.
>
> But R is not a parameter on the function itself. It comes from the outside template.
>
> Move it to the inside template, rewrite it as:
>
> 	auto a(S, R)(auto ref R i) {
> 		return cast(S)i*2;
> 	}
>
> and you should get further.

But surely nested template should be able to access outer template's parameter.

On Sunday, 21 June 2015 at 01:26:51 UTC, Adam D. Ruppe wrote:
> On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
>> auto ref R) is indeed a template function, so I don't understand.
>
> But R is not a parameter on the function itself. It comes from the outside template.
>
> Move it to the inside template, rewrite it as:
>
> 	auto a(S, R)(auto ref R i) {
> 		return cast(S)i*2;
> 	}
>
> and you should get further.

Also the same error persists even if I change 'a' to

auto a(S)(auto ref S i)

On 6/20/15 10:26 PM, Yuxuan Shui wrote:
> On Sunday, 21 June 2015 at 01:26:51 UTC, Adam D. Ruppe wrote:
>> On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
>>> auto ref R) is indeed a template function, so I don't understand.
>>
>> But R is not a parameter on the function itself. It comes from the
>> outside template.
>>
>> Move it to the inside template, rewrite it as:
>>
>>     auto a(S, R)(auto ref R i) {
>>         return cast(S)i*2;
>>     }
>>
>> and you should get further.
>
> But surely nested template should be able to access outer template's
> parameter.

If you have to instantiate it to see how to instantiate it, it doesn't work. General rule of thumb on templates. IFTI only works at one level.

-Steve

On Monday, 22 June 2015 at 13:49:21 UTC, Steven Schveighoffer wrote:
> On 6/20/15 10:26 PM, Yuxuan Shui wrote:
>> On Sunday, 21 June 2015 at 01:26:51 UTC, Adam D. Ruppe wrote:
>>> On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
>>>> auto ref R) is indeed a template function, so I don't understand.
>>>
>>> But R is not a parameter on the function itself. It comes from the
>>> outside template.
>>>
>>> Move it to the inside template, rewrite it as:
>>>
>>>     auto a(S, R)(auto ref R i) {
>>>         return cast(S)i*2;
>>>     }
>>>
>>> and you should get further.
>>
>> But surely nested template should be able to access outer template's
>> parameter.
>
> If you have to instantiate it to see how to instantiate it, it doesn't work. General rule of thumb on templates. IFTI only works at one level.
>
> -Steve

I can't see any IFTI in my code, can you point it out?

On 6/22/15 1:37 PM, Yuxuan Shui wrote:
> On Monday, 22 June 2015 at 13:49:21 UTC, Steven Schveighoffer wrote:
>> On 6/20/15 10:26 PM, Yuxuan Shui wrote:
>>> On Sunday, 21 June 2015 at 01:26:51 UTC, Adam D. Ruppe wrote:
>>>> On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
>>>>> auto ref R) is indeed a template function, so I don't understand.
>>>>
>>>> But R is not a parameter on the function itself. It comes from the
>>>> outside template.
>>>>
>>>> Move it to the inside template, rewrite it as:
>>>>
>>>>     auto a(S, R)(auto ref R i) {
>>>>         return cast(S)i*2;
>>>>     }
>>>>
>>>> and you should get further.
>>>
>>> But surely nested template should be able to access outer template's
>>> parameter.
>>
>> If you have to instantiate it to see how to instantiate it, it doesn't
>> work. General rule of thumb on templates. IFTI only works at one level.
>>
> I can't see any IFTI in my code, can you point it out?

Sorry, I misunderstood what is happening here. I don't know what the rules are for auto ref for nested template functions, or for explicit instantiation.

-Steve

On Sunday, 21 June 2015 at 02:37:59 UTC, Yuxuan Shui wrote:
> On Sunday, 21 June 2015 at 01:26:51 UTC, Adam D. Ruppe wrote:
>> On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
>>> auto ref R) is indeed a template function, so I don't understand.
>>
>> But R is not a parameter on the function itself. It comes from the outside template.
>>
>> Move it to the inside template, rewrite it as:
>>
>> 	auto a(S, R)(auto ref R i) {
>> 		return cast(S)i*2;
>> 	}
>>
>> and you should get further.
>
> Also the same error persists even if I change 'a' to
>
> auto a(S)(auto ref S i)

I'm not reproducing in DMD 2.067:

    void main() {
        int m;
        a!int(m);
    }
    template a(R) {
        auto a(S)(auto ref S i) {
        }
    }

Note that since template 'a' and function template 'a' have the same name you're invoking IFTI, otherwise the code would look like:

    void main() {
    	int m;
    	z!int.a(m);
    }
    template z(R) {
        auto a(S)(auto ref S i) {
        }
    }

On Saturday, 20 June 2015 at 01:50:11 UTC, Yuxuan Shui wrote:
> Try to compile this code snippet:
>
> import std.traits;
> template a(R) {
> 	auto a(S)(auto ref R i) {
> 		return cast(S)i*2;
> 	}
> }
> template ReturnTypeEx(alias A, B) {
> 	alias ReturnTypeEx = ReturnType!(A!B);
> }
> template b(alias R) {
> 	int b(S)(S i) {
> 		alias Ra = ReturnTypeEx!(R, S);
> 		return cast(int)R!S(i);
> 	}
> }
> void main() {
> 	alias bb = b!(a!ulong);
> 	pragma(msg, ReturnTypeEx!(bb, int));
> }
>
> DMD reports:
> sadf.d(3): Error: auto can only be used for template function parameters
> sadf.d(8): Error: template instance sadf.a!ulong.A!int error instantiating
> sadf.d(12):        instantiated from here: ReturnTypeEx!(a, int)
> sadf.d(8):        instantiated from here: A!int
> sadf.d(18):        instantiated from here: ReturnTypeEx!(b, int)
> sadf.d(18):        while evaluating pragma(msg, ReturnTypeEx!(b, int))
>
> But a(S)(auto ref R) is indeed a template function, so I don't understand.

After more experimenting, here's what I got. This time no nested templates are involved, this seems more likely an 'auto ref' bug.

import std.traits, std.range;

void a(S)(auto ref S i) { }
void b(S)(auto ref S i) if (isInputRange!S) { }
void c(S)(ref S i) if (isInputRange!S) { }

void devil(alias S)() { }

void main() {
    a!string(""); //Works  <--- This line affects the result of devil!(a!string)
    b!string(""); //Works

    //Next line is weird, it:
    //1. Err, 'auto ref can only be used with template function', if 'a' is not
    //   instantiated with 'a!string' first
    //2. Works, if 'a!string' is done first
    alias x = devil!(a!string);

    alias xx = devil!(b!string); //Err, template doesn't match

    alias xxx = devil!(c!string); //Works
}

I'm using DMD 2.067.1