Hello everyone,

I'm the author of DIP 1023 [1], [2].

I'll try to keep this short to save you time by only mentioning the important
parts (and those are a lot anyway).
Feel free to ask me to elaborate on anything that it's not clear.

So, first of all, D can't cope with alias templates used as function
parameters (in template functions). For example:

struct TypeTemplate(T) {}
alias AliasTemplate(T) = TypeTemplate!T;
void functionTemplate(T)(AliasTemplate!T arg) {}

void main()
{
    AliasTemplate!int inst;
    functionTemplate(inst); /* "cannot deduce function from argument types !()(TypeTemplate!int)" */
}

DIP 1023 was introduced because the reasoning was that this is not simply
a bug and the feature was in fact, under-specified.
That is, a bug is when we have the form "When I do X, Z should happen" but
it doesn't. "Under-specified" means that "When I do X, it's not even clear
that Z should happen in the first place".
So, this is where the compiler writer requests a specification addition.

My position is that while I believe that Alias Templates are in general
under-specified in D, adding any more specification won't help
DMD writers _in this specific issue_.

Let me elaborate: Let's say that we do have a good enough specification,
like C++'s.

With the help of Nicholas Wilson who asked on reddit, I was redirected
to the C++ standard [3] in the sections 17.6.7 Alias Templates
and 17.5 Type Equivalence.

I think having such a specification won't help anyone here.
To be even more specific, as far as I can understand,
the issue at hand is handled in the C++ standard with 3 sentences (point 2,
section 17.6.7):

"When a template-id refers to the specialization of an alias template, it is equivalent to the associated type obtained by substitution of its template-arguments for the template-parameters in the type-id of the alias
template".

And this something that we already know, yet can't solve the issue.

What's your opinion on this ?

Btw, please consider that this is not my area of expertise. :)

Best regards,
Stefanos Baziotis

[1] https://github.com/dlang/DIPs/pull/176
[2] https://forum.dlang.org/post/dnyqxmgdazczwmmvayjx@forum.dlang.org
[3] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4713.pdf

On Sunday, 13 October 2019 at 10:51:52 UTC, Stefanos Baziotis wrote:
> Hello everyone,
>
> I'm the author of DIP 1023 [1], [2].
>
> I'll try to keep this short to save you time by only mentioning the important
> parts (and those are a lot anyway).
> Feel free to ask me to elaborate on anything that it's not clear.
>
> So, first of all, D can't cope with alias templates used as function
> parameters (in template functions). For example:
>
> struct TypeTemplate(T) {}
> alias AliasTemplate(T) = TypeTemplate!T;
> void functionTemplate(T)(AliasTemplate!T arg) {}
>
> void main()
> {
>     AliasTemplate!int inst;
>     functionTemplate(inst); /* "cannot deduce function from argument types !()(TypeTemplate!int)" */
> }
>
> DIP 1023 was introduced because the reasoning was that this is not simply
> a bug and the feature was in fact, under-specified.
> That is, a bug is when we have the form "When I do X, Z should happen" but
> it doesn't. "Under-specified" means that "When I do X, it's not even clear
> that Z should happen in the first place".
> So, this is where the compiler writer requests a specification addition.
>
> My position is that while I believe that Alias Templates are in general
> under-specified in D, adding any more specification won't help
> DMD writers _in this specific issue_.
>
> Let me elaborate: Let's say that we do have a good enough specification,
> like C++'s.
>
> With the help of Nicholas Wilson who asked on reddit, I was redirected
> to the C++ standard [3] in the sections 17.6.7 Alias Templates
> and 17.5 Type Equivalence.
>
> I think having such a specification won't help anyone here.
> To be even more specific, as far as I can understand,
> the issue at hand is handled in the C++ standard with 3 sentences (point 2,
> section 17.6.7):
>
> "When a template-id refers to the specialization of an alias template, it is equivalent to the associated type obtained by substitution of its template-arguments for the template-parameters in the type-id of the alias
> template".
>
> And this something that we already know, yet can't solve the issue.
>
> What's your opinion on this ?
>
> Btw, please consider that this is not my area of expertise. :)
>
> Best regards,
> Stefanos Baziotis
>
> [1] https://github.com/dlang/DIPs/pull/176
> [2] https://forum.dlang.org/post/dnyqxmgdazczwmmvayjx@forum.dlang.org
> [3] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4713.pdf

This is because DMD's current implementation of template argument deduction isn't advanced enough to cover this case, but it could be improved to cover it.

In your example, in "functionTemplate(inst);" inst has its type resolved to TypeTemplate!T and DMD's argument deduction tries matching the identifier "TypeTemplate" to the identifier "AliasTemplate", which fails. The argument deduction function in DMD doesn't know/check that AliasTemplate!T would actually resolve to TypeTemplate!T.

Regarding C++, C++ template argument deduction is more "sophisticated" but also has its limits, maybe to keep the complexity of the implementation at an acceptable level.
An example of C++ template function that doesn't work with the rules of C++ template argument deduction is std::forward<T>(std::remove_reference_t<T>& t) (see https://en.cppreference.com/w/cpp/utility/forward), C++ compilers cannot deduce T by matching the argument type to std::remove_reference<T>. Thus std::forward can only be used with an explicit template argument.

The C++ equivalent of your example does work, but alias templates are different in C++ and D. In C++ they only alias types, while in D there is no concept of alias template, your AliasTemplate may have one or more overloads that instantiate a variable or some other kind of symbol completely unrelated to aliases.
So C++ has it easier and the paragraph you quoted from the spec makes argument deduction possible by first substituting the function parameter by a "dummy instantiation of AliasTemplate<T>". A similar solution wouldn't be easy to implement in DMD for the aforementioned reasons, but it may be do-able.

IMHO if not done yet you should submit a bugzilla issue with your example and perhaps dive into DMD's code and try figuring out a solution.

On Sunday, 13 October 2019 at 15:06:11 UTC, Elie Morisse wrote:
> In C++ they only alias types, while in D there is no concept of alias template,

By that I meant that:

alias SomeAlias(T) = ...;

is syntactic sugar for:

template SomeAlias(T) { alias SomeAlias = ...; }


And unlike in C++ SomeAlias might have overloads that instantiate something completely different :

template SomeAlias(T : char) { enum SomeAlias = 987; }
template SomeAlias(T : int) { void SomeAlias() {} }

On Sunday, 13 October 2019 at 10:51:52 UTC, Stefanos Baziotis wrote:
> [1] https://github.com/dlang/DIPs/pull/176
> [2] https://forum.dlang.org/post/dnyqxmgdazczwmmvayjx@forum.dlang.org

I read your DIP and the review thread.

> if (arg is templateInstance && arg.TD is aliasTemplateDeclaration) {

Checking that a TemplateDeclaration is an "alias template declaration" is easy : TemplateDeclaration.onemember && TemplateDeclaration.onemember.isAliasDeclaration()

However since they may be more than one TemplateDeclaration overload, your templateInstance cannot exist yet and you don't know which TemplateDeclaration overload should be probed..

A possible solution like C++'s, but taking into account the multiplicity of TemplateDeclaration overloads would be:

 - If the first round of arg deduction fails, try transforming the TypeFunction by substituting each function parameter that failed matching with a dummy instance from each TemplateDeclaration overload that contains an alias (one substitution attempt per overload) and redo the arg deduction. For each function parameter, no more than one TemplateDeclaration overload must match or else it's ambiguous.
 - If every combination of "one substitution per param" fails, keep substituting the remaining non-matching already substituted function parameters until there's no deeper alias template (i.e no more alias Temp(T) = SomeOtherAliasTemp(T)).

Downside : there may be A LOT of possible combinations.

On Sunday, 13 October 2019 at 16:28:00 UTC, Elie Morisse wrote:
> Downside : there may be A LOT of possible combinations.

Actually function template argument deduction is done parameter by parameter (left to right), so the number of possible combinations shouldn't be too high.

On Sunday, 13 October 2019 at 15:06:11 UTC, Elie Morisse wrote:
>
> This is because DMD's current implementation of template argument deduction isn't advanced enough to cover this case, but it could be improved to cover it.
>

Yes, the problem though is that DMD developers could not cover it,
so the DIP was introduced to help.

> Regarding C++, C++ template argument deduction is more "sophisticated" but also has its limits, maybe to keep the complexity of the implementation at an acceptable level.
> An example of C++ template function that doesn't work with the rules of C++ template argument deduction is std::forward<T>(std::remove_reference_t<T>& t) (see https://en.cppreference.com/w/cpp/utility/forward), C++ compilers cannot deduce T by matching the argument type to std::remove_reference<T>. Thus std::forward can only be used with an explicit template argument.

I didn't know that, interesting.

>
> The C++ equivalent of your example does work, but alias templates are different in C++ and D. In C++ they only alias types, while in D there is no concept of alias template, your AliasTemplate may have one or more overloads that instantiate a variable or some other kind of symbol completely unrelated to aliases.
> So C++ has it easier and the paragraph you quoted from the spec makes argument deduction possible by first substituting the function parameter by a "dummy instantiation of AliasTemplate<T>". A similar solution wouldn't be easy to implement in DMD for the aforementioned reasons, but it may be do-able.

I have some little experience with DMD, but not enough to
answer this fully. From a draft implementation I did, it seems
that for the simple case, it should be doable.
However, what you mentioned with the overloads etc. is interesting,
I'll refer to it again below.

>
> IMHO if not done yet you should submit a bugzilla issue with your example and perhaps dive into DMD's code and try figuring out a solution.

You can check the DIP I referenced above [1]. There are links to a draft
implementation and a lot of bugzilla reports.

> And unlike in C++ SomeAlias might have overloads that instantiate something completely different :
> 
> template SomeAlias(T : char) { enum SomeAlias = 987; }
> template SomeAlias(T : int) { void SomeAlias() {} }

This is interesting in that a specification for alias templates as C++'s
may not be enough and an addition may actually help. TBH, I don't think though.
Because DMD currently can't handle even the simplest case.
I left the implementation and started the DIP when I stumbled upon problems
that didn't have to do with aliases but with template instantiation.
This is something that definitely should be able to be solved with the
current specification, yet it doesn't.

> A possible solution like C++'s, but taking into account the multiplicity of TemplateDeclaration overloads would be:

Some of the things you mentioned are done but I don't remember wholly.
However, I get the idea.
Currently, I don't have time to work on the implementation though.
Consider that this was supposed to have ended by the end of May.
Unfortunately, it's unclear how much time will I have to do only the DIP work.
So basically, I can focus only on the specification side of things and
leave the implementation reasoning (i.e. whether the DIP will help with that,
how it can be done etc.) to the DMD developers.

Thanks for your input!
- Stefanos

[1] https://github.com/dlang/DIPs/pull/176